Fix provider url which was always empty

Modify GerritConnection to have the gerritFrontEndUrl so it can create
provider with a url instead of creating it with an empty url.

Change-Id: I58c82892668567afc77b78f2cf2d0c616124075f
Fix changeIdAnnotator when annotating a build configured with anyServer

ChangeIdAnnotator was using the serverName configured at the job level
to find the front end url of the gerrit server but it is failing when
job is configured with "Any Server" as server name.

Use the gerritCause of the build instead to find the url of the gerrit
server.

Change-Id: I08332e788c5f23417425599d2ab60fb41fde0f92
Merge pull request #130 from hugares/change-id-annotator-fix

Fix ChangeIdAnnotator for "Any Server" option