set user/group under systemd

[graphaelli]

if not root. apm-server does not run as root by default as of 7.0.0

[graphaelli]

single unrelated & known failure on CI:

14:41:13 command [go test -race -tags integration -cover -coverprofile /tmp/gotestcover-282605320 github.com/elastic/beats/libbeat/outputs/transport/transptest]: exit status 1
14:41:13 2019/04/12 18:41:13 [ERR] socks: Failed to handle request: Failed to resolve destination 'invalid.dns.fqdn-unknown.invalid': lookup invalid.dns.fqdn-unknown.invalid on 127.0.0.11:53: no such host
14:41:13 2019/04/12 18:41:13 [ERR] socks: Failed to handle request: Failed to resolve destination 'invalid.dns.fqdn-unknown.invalid': lookup invalid.dns.fqdn-unknown.invalid on 127.0.0.11:53: no such host
14:41:13 2019/04/12 18:41:13 [ERR] socks: Failed to handle request: readfrom tcp 127.0.0.1:56454->127.0.0.1:44435: splice: broken pipe
14:41:13 2019/04/12 18:41:13 [ERR] socks: Failed to handle request: readfrom tcp 127.0.0.1:53434->127.0.0.1:38455: splice: broken pipe
14:41:13 PASS
14:41:13 coverage: 47.5% of statements
14:41:13 ==================
14:41:13 WARNING: DATA RACE
14:41:13 Read at 0x00c000104843 by goroutine 18:
14:41:13   testing.(*common).logDepth()
14:41:13       /usr/local/go/src/testing/testing.go:621 +0x94
14:41:13   testing.(*common).Log()
14:41:13       /usr/local/go/src/testing/testing.go:614 +0x78
14:41:13   github.com/elastic/beats/libbeat/outputs/transport/transptest.newSOCKS5Proxy.func1()
14:41:13       /go/src/github.com/elastic/beats/libbeat/outputs/transport/transptest/testing_test.go:57 +0x9f
14:41:13 
14:41:13 Previous write at 0x00c000104843 by goroutine 19:
14:41:13   testing.tRunner.func1()
14:41:13       /usr/local/go/src/testing/testing.go:856 +0x354
14:41:13   testing.tRunner()
14:41:13       /usr/local/go/src/testing/testing.go:869 +0x17f
14:41:13 
14:41:13 Goroutine 18 (running) created at:
14:41:13   github.com/elastic/beats/libbeat/outputs/transport/transptest.newSOCKS5Proxy()
14:41:13       /go/src/github.com/elastic/beats/libbeat/outputs/transport/transptest/testing_test.go:54 +0x261
14:41:13   github.com/elastic/beats/libbeat/outputs/transport/transptest.TestTransportClosedOnWriteReadError()
14:41:13       /go/src/github.com/elastic/beats/libbeat/outputs/transport/transptest/testing_test.go:138 +0x6d
14:41:13   testing.tRunner()
14:41:13       /usr/local/go/src/testing/testing.go:865 +0x163
14:41:13 
14:41:13 Goroutine 19 (finished) created at:
14:41:13   testing.(*T).Run()
14:41:13       /usr/local/go/src/testing/testing.go:916 +0x65a
14:41:13   testing.runTests.func1()
14:41:13       /usr/local/go/src/testing/testing.go:1157 +0xa8
14:41:13   testing.tRunner()
14:41:13       /usr/local/go/src/testing/testing.go:865 +0x163
14:41:13   testing.runTests()
14:41:13       /usr/local/go/src/testing/testing.go:1155 +0x523
14:41:13   testing.(*M).Run()
14:41:13       /usr/local/go/src/testing/testing.go:1072 +0x2eb
14:41:13   main.main()
14:41:13       _testmain.go:94 +0x334
14:41:13 ==================
14:41:13 panic: Log in goroutine after TestTransportClosedOnWriteReadError has completed
14:41:13 
14:41:13 goroutine 134 [running]:
14:41:13 testing.(*common).logDepth(0xc000104800, 0xc00001d180, 0x3d, 0x3)
14:41:13 	/usr/local/go/src/testing/testing.go:634 +0x51a
14:41:13 testing.(*common).log(...)
14:41:13 	/usr/local/go/src/testing/testing.go:614
14:41:13 testing.(*common).Log(0xc000104800, 0xc000045fa0, 0x1, 0x1)
14:41:13 	/usr/local/go/src/testing/testing.go:642 +0x79
14:41:13 github.com/elastic/beats/libbeat/outputs/transport/transptest.newSOCKS5Proxy.func1(0xc0002c53c0, 0x9a4e60, 0xc00008c230, 0xc000104800)
14:41:13 	/go/src/github.com/elastic/beats/libbeat/outputs/transport/transptest/testing_test.go:57 +0xa0
14:41:13 created by github.com/elastic/beats/libbeat/outputs/transport/transptest.newSOCKS5Proxy
14:41:13 	/go/src/github.com/elastic/beats/libbeat/outputs/transport/transptest/testing_test.go:54 +0x262
14:41:13 FAIL	github.com/elastic/beats/libbeat/outputs/transport/transptest	1.268s