use maven3 instead of maven2

I can't seem to run the plugin-in in dev mode using maven2 against any jenkins version greater
than 1.460.  I always get a java.lang.NullPointerException when running with maven 'hpi:run'
command.  I don't get this error when using maven3 and maven3 is happy running against
any version of jenkins.

Also I noticed that all of openstack projects are using maven3, except for this project so i thought i
would make it consistant.

Change-Id: Ib39e25f76706e37613a5fb9e056b131b1879bb88
Reviewed-on: https://review.openstack.org/22643
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins