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

shad7/oslo.config,JioCloud/oslo.config,citrix-openstack-build/oslo.config,citrix-openstack/build-oslo.config,magic0704/oslo.config,tianshan/oslo.config,metacloud/oslo.config,varunarya10/oslo.config