feat(promtail): initContainers (#655)

* fix(promtail/targets): remove dependency on prometheus/relabel

`filetargetmanager.go` had a dependency on prometheus/relabel, a package that
has been removed in favor of prometheus/pkg/relabel.
This converts the code to the new package, to allow the prometheus vendor be
updated to master

* chore(vendor): update prometheus vendor

Updates prometheus vendor to current master, to add support for InitContainers
in kubernetes service discovery

* chore(vendor): do dep's homework

Gives dep hints on how to resolve the vendor so that it work

* fix(promtail/targets): non-nil check

Accidentally checked against the wrong labelSet, this one can never get nil