ML2 support

Divided CONFIG_NEUTRON_L2_PLUGIN parameter to CONFIG_NEUTRON_L2_PLUGIN
and CONFIG_NEUTRON_L2_AGENT to enable separate installation of agent
in case of ML2 plugin installation.

Implemented ML2 plugin installation using classes from puppet-neutron
module.

Fixes: rhbz#1017144

Change-Id: I6b769b7aec894f2ef87d9b33dd02fbc20abb9e3a
(cherry picked from commit 7be825d7b3fcd5be526ff9920516492c3c1a0e6e)
Merge "ML2 support" into havana