Implement LBaaS object model v2

This change implements the LBaaS v2 object model changes necessary for
python-neutronclient. It includes new objects loadbalancer and listener,
as well as new implementations for pool, member, and healthmonitor.
While many constructs are the same as in v1, many are not, and have
therefore been namespaced under lb/v2.

This client now supports both the v1 and v2 implementations
simultaneously. v1 commands retain their original form:
neutron lb-<object>-<command>

and v2 commands take the form:
neutron lbaas-<object>-<command>

Change-Id: I970b1de39d0dd8872459a584dd1f5c9110796ac9
Partially-implements: blueprint lbaas-api-and-objmodel-improvement
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