Set mavenName for Jenkins Maven jobs.

Set mavenName to Maven3 (the current default for all jobs) on all jobs
but the gearman-plugin job. Jenkins plugins need to be built by maven2
so set the mavenName for gearman-plugin to Maven2. This maintains
current maven settings for all projects but the broken project.

Change-Id: I15334ea48280b9389e3660c69090b58068dbf9c3
Reviewed-on: https://review.openstack.org/19760
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins