heat engine : Disable stack rollback by default

Until the event-persistence BP is implemented, we should
disable stack rollback by default and make it opt-in.

fixes bug 1131303

Change-Id: I101e108c3959c4b7b4cfc68449ba2b989b157626
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