Use a proper type for unmarshalling config yaml

It's not possible to use the upstream type from the Alertmanager
project completely, as it will obfuscate any secrets in the yaml that
it creates, which we don't want. See the following issue for more
info: prometheus/alertmanager#1985

go.mod

@@ -23,6 +23,7 @@ require (
 	github.com/prometheus-operator/prometheus-operator/pkg/apis/monitoring v0.0.0-00010101000000-000000000000
 	github.com/prometheus/alertmanager v0.20.0
 	github.com/prometheus/client_golang v1.6.0
+	github.com/prometheus/common v0.10.0
 	github.com/prometheus/prometheus v1.8.2-0.20200609102542-5d7e3e970602
 	github.com/stretchr/testify v1.5.1
 	github.com/thanos-io/thanos v0.11.0

go.sum

@@ -57,6 +57,7 @@ github.com/Azure/go-autorest/autorest/validation v0.2.0/go.mod h1:3EEqHnBxQGHXRY
 github.com/Azure/go-autorest/autorest/validation v0.2.1-0.20191028180845-3492b2aff503/go.mod h1:3EEqHnBxQGHXRYq3HT1WyXAvT7LLY3tl70hw6tQIbjI=
 github.com/Azure/go-autorest/logger v0.1.0/go.mod h1:oExouG+K6PryycPJfVSxi/koC6LSNgds39diKLz7Vrc=
 github.com/Azure/go-autorest/tracing v0.5.0/go.mod h1:r/s2XiOKccPW3HrqB+W0TQzfbtp2fGCgRFtBroKn4Dk=
+github.com/BurntSushi/toml v0.3.1 h1:WXkYYl6Yr3qBf1K79EBnL4mak0OimBfB0XUf9Vl28OQ=
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
 github.com/BurntSushi/xgb v0.0.0-20160522181843-27f122750802/go.mod h1:IVnqGOEym/WlBOVXweHU+Q+/VP0lqqI8lqeDx9IjBqo=
 github.com/DATA-DOG/go-sqlmock v1.3.3/go.mod h1:f/Ixk793poVmq4qj/V1dPUg2JEAKC73Q5eFN3EC/SaM=
@@ -1247,6 +1248,7 @@ honnef.co/go/tools v0.0.0-20190106161140-3f1c8253044a/go.mod h1:rf3lG4BRIbNafJWh
 honnef.co/go/tools v0.0.0-20190418001031-e561f6794a2a/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
 honnef.co/go/tools v0.0.0-20190523083050-ea95bdfd59fc/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
 honnef.co/go/tools v0.0.1-2019.2.3/go.mod h1:a3bituU0lyd329TUQxRnasdCoJDkEUEAqEt0JzvZhAg=
+honnef.co/go/tools v0.0.1-2020.1.3 h1:sXmLre5bzIR6ypkjXCDI3jHPssRhc8KD/Ome589sc3U=
 honnef.co/go/tools v0.0.1-2020.1.3/go.mod h1:X/FiERA/W4tHapMX5mGpAtMSVEeEUOyHaw9vFzvIQ3k=
 howett.net/plist v0.0.0-20181124034731-591f970eefbb/go.mod h1:vMygbs4qMhSZSc4lCUl2OEE+rDiIIJAIdR4m7MiMcm0=
 k8s.io/api v0.0.0-20190620084959-7cf5895f2711/go.mod h1:TBhBqb1AWbBQbW3XRusr7n7E4v2+5ZY8r8sAMnyFC5A=