yeti should read plan ENV variable if service_manifest does not
contain plan configure

Change-Id: Ife227cd4cf4a39b8830ada4d21c4f5a684f62f24
Reviewed-on: http://reviews.cloudfoundry.org/13654
Tested-by: CI Master <cf-ci@rbcon.com>
Reviewed-by: Frank Lu <flu@vmware.com>
Reviewed-by: Nicholas Kushmerick <nicholask@vmware.com>