make engine api handle string or bool disable_rollback

Allow API's to pass string or bool format True/False flag
for the disable_rollback argument to the engine RPC API

Change-Id: I1886263ac2487c4cb36c8bd93fd1d09b04abe813
heat api : Handle DisableRollback parameter in cfn API

Previously we ignored DisableRollback for create/update,
pass it to the engine since the engine API now supports it

blueprint stack-rollback

Change-Id: I926d843a88449fa61b665bd1b47946724f6d2ce5