Change create flavor to use default arguments

Most of the arguments required by the create flavor API can have
reasonable defaults and therefore can be made optional in the CLI.
This brings create flavor in line with the documented args in the
wiki at https://wiki.openstack.org/wiki/UnifiedCLI/Mapping#flavor

Change-Id: Iecb3baf72f9dc3981742ff7989780894e37921c9
Merge "Change create flavor to use default arguments"