rgw: user can specify 'rgw port' to listen on a tcp port.

'rgw socket path' overrides 'rgw port'.
An 'rgw host' can be set to listen on a specific ip (default is 0.0.0.0)

Signed-off-by: Guilhem Lettron <guilhem.lettron@youscribe.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>