Add -S option to support subgraph execution. This would allow a user
to pick which tasks to stop on.
Example:
fuel2 graph execute -S keystone-db:openstack-controller/1
will start with all keystone-db tasks and end at openstack-controller at
node-1
DocImpact
Change-Id: I42f9caf2e9c18072a2ae0447df2ba3500f687d5e
Closes-bug: 1612616