Use neutron-lbaas conditionally.

Since lbaas is currently a moving target, it is desirable
to release it only where required, to avoid painful
upgrades / migrations later.

This change causes neutron-lbaas agent to be configured and
started only if `neutron.enable_lbaas` var is true.
Merge pull request #311 from blueboxgroup/conditional-lbaas

Use neutron-lbaas conditionally.