Check compulsory flavor create parameters exist

Check that the name, ram, vcpu and disk parameters exist
in the request for flavor creation in order to be able to
return a more informative error message to a user when they
are not supplied.

Change-Id: I7fd235f8b57e5ecba37e50d6f0ce0a3866c1cd55
Closes-Bug: 1253525
Merge "Check compulsory flavor create parameters exist"