Renaming flavorRef to flavor_ref

This patch changes all references to the JSON property "flavorRef" to
"flavor_ref". It also changes the corresponding python variable name from
"flavorRef" to "flavor_ref" for a) consistency with the JSON property, and
b) per PEP8's prescribed naming conventions (http://legacy.python.org/dev/peps/pep-0008/#prescriptive-naming-conventions).

Change-Id: I803dc724a44eabfa50fe1a5f85d7b53005a6b47a