Ansible launcher: Add graceful stop command
Ansible launcher: Add graceful stop command

Stops accepting new nodes/jobs, releases idle nodes, waits for
running jobs to finish, then exits.

Change-Id: I8d5dfc101fce0d3d6e1ebd1884ec29cbb1de7841

wikimedia/integration-zuul,gooddata/zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul