mgcp-client vty: use name 'mgw' instead of 'mgcpgw'

'mgcpgw' was a working title for the osmo-mgw. Before this takes hold out
there, let's rename the VTY commands to 'mgw'. I'd rather have some local
fallout in our testing environments now than drag the stupid name along.

Keep deprecated 'mgcpgw' commands for backwards compat.

Change-Id: I1d43d42929dc9162e57640499526fb7cadbcfbe6
1 file changed