Prevents flavors to be deleted when reusing a name

Adds an extra check to the EditFlavor form
in order to avoid losing the flavor if an
existing name is setted as the new name.

This control is needed because the current
edit implementation is based in a
delete-create action.

Change-Id: I4251c64fa4719831a1ed949bf95349a350dd303d
Fixes: bug #1133254
Merge "Prevents flavors to be deleted when reusing a name" into stable/folsom