radosgw-admin: various multiregion related fixes and changes

Go to the master zone in the master region for radosgw-admin
operations. Trigger metadata sync. Other fixes.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
agent name should also contain port number

otherwise multiple agents per machines will have the same id

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>