Add ability to specify mavenName for maven jobs.

Previously maven jobs did not specify a maven name so which maven
install Jenkins treated as default was used for all JJB maven jobs. Add
the ability to set a maven install explicitly so that multiple maven
installs may be used.

Change-Id: I6a26635c31c5504b855063e1d4f6fa53132f26b5
Reviewed-on: https://review.openstack.org/19756
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins