heat clients : Change --disable-rollback to --enable-rollback

Since the engine default is now rollback disabled, flip this
switch so the user can opt-in to rollback functionality via
the --enable-rollback cli option to heat-cfn/heat-boto

ref bug 1131303

Change-Id: I3965de9a2beb510d5623d0c7cd8ff3aea9d4364a