Implement Flavor Extra-specs Key/Value Pairs

Add flavor set_keys function to nova_flavor resource.

Change-Id: I5d46ace4d729a2c4a1b136265b146417e7ad2812
Makefile option to build xml docs

Change-Id: I15316f7a8e3fef679c9b322ef34d94848715b206
Merge "Implement Flavor Extra-specs Key/Value Pairs"
Merge "Makefile option to build xml docs"