Big Switch: fix capabilities retrieval code

References the raw REST response in the capabilities
parsing code so json.loads doesn't get an object
that may already be decoded and fail.

Conflicts:
	neutron/tests/unit/bigswitch/test_capabilities.py
	neutron/tests/unit/bigswitch/test_servermanager.py

Closes-Bug: #1326173
Change-Id: Ia3179b7499f35a6ab2e9ce1631ab15ed27d64647
Merge "Big Switch: fix capabilities retrieval code" into stable/icehouse