Add /services endpoint

[yvrhdn]

What this PR does:
Add /services which prints a plaintext list of all the services in the service map with their status and optional failure case.

$ curl  localhost:3201/services
memberlist-kv: Running
querier: Running
ring: Running
server: Running
store: Running

Cortex has a similar endpoint which returns HTML, but I decided to keep this simple and plaintext.
https://github.com/cortexproject/cortex/blob/master/pkg/cortex/status.go

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[joe-elliott]

nice!

i was wanting this yesterday when the compactors were failing to start for some reason.