contint: apply maven settings on labs slaves

The production slaves run with the jenkins-slave user which has a maven
setting to point maven to our web proxy.  The slaves (running jobs with
jenkins-deploy) are missing that configuration which cause issue
whenever maven attempt to access URLs hosted in labs (such as
toolslabs.wmflabs.org).

Add a required 'homedir' parameter to contint::maven_webproxy. Set it
accordingly for prod and slave labs.

Change-Id: I433366ea51740c9a8256a755e8171fab90fb80ee
Merge "contint: apply maven settings on labs slaves" into production