Use the main-debian8 variant of golang-crossbuilder to build Filebeat
packages for linux/amd64 and linux/368. Previously a debian7 image was
used, but debian7 does not have the systemd libraries needed for journald.

Debian 8 uses glibc 2.19 and debian 7 uses 2.13. We used debian7 because
the binaries produced worked on RHEL 6 which has glibc 2.12.

To ensure that the Filebeat binaries for linux/amd64 and linux/386 continue
to work on RHEL 6 the build process will check the glibc requirement to
ensure it is <=2.12.

To keep the linked glibc version compatible with RHEL 6 I had to upgrade to
github.com/dgraph-io/badger/v3 from v2. v3 removed a bunch of cgo usages which
reduced the need on various libc functions.

badger adopted golang/glog for logging. This library is adds global CLI
flags that conflict with Beats own logging CLI flags (like '-v'). This is
a common problem for glog (k8s encountered this). To address the problem
I forked golang/glog and made the flag.FlagSet used by the library configurable.