Add caching to openstack.common.cfg
Add caching to openstack.common.cfg

Speedup of 'nova list' benchmark by up to 40%, eliminates 3 lines in
top-10 cProfile methods.

Change-Id: I2d4636f94d88b4a7e38d1565fdd4d1b8a89e560e

JioCloud/pbr,redhat-openstack/pbr,emonty/oslo-packaging,citrix-openstack-build/pbr,varunarya10/pbr,citrix-openstack-build/pbr,redhat-openstack/pbr,varunarya10/pbr,openstack-dev/pbr,citrix-openstack-build/pbr,klmitch/pbr,openstack-dev/pbr,klmitch/pbr,WiFast/pbr,emonty/oslo-packaging,JioCloud/pbr,WiFast/pbr,openstack-dev/pbr