Readme : Change swarm_manager to swarm_master
Readme : Change swarm_manager to swarm_master

When swarm bays are being created, the bay api address is not
being set. This is because in the template definition, the
api_address attribute is being set on the swarm_manager, not
the swarm_master.

Change updates the Swarm readme.

Change-Id: Iba7dc06fd262d7e0976db81cfceae487ec02fc33
Closes-Bug: #1496192

dimtruck/magnum,eshijia/magnum,openstack/magnum,ArchiFleKs/magnum,jay-lau/magnum,eshijia/magnum,openstack/magnum,Tennyson53/magnum,ArchiFleKs/magnum,dimtruck/magnum,Tennyson53/magnum