Add Sonar Plugin support.

The Sonar Plugin provides both a builder and a publisher,
the later targeting Maven projects.

This commit adds support for the publisher, which is straightforward
to configure: no option is required, the plugin finds suitable
defaults by itself.

Several parameters can be provided though, which are transferred
as is to either Sonar or Maven. Analysis can also be skipped under
certain conditions.

Change-Id: I30744d0961d623cabf7c0780ab2f0d4db2cd9295
Reviewed-on: https://review.openstack.org/25073
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins