Docs: specify --docker-volume-size for swarm bay
Docs: specify --docker-volume-size for swarm bay

If this option is not specified, the default is 25GB which is too big
to fail the stack creation.

Change-Id: I75fca1ab150f2df76c2464647c3d4f73dd7463f7
Closes-Bug: #1520308

jay-lau/magnum,openstack/magnum,ArchiFleKs/magnum,openstack/magnum,ArchiFleKs/magnum