Make the MGW pool configurable in the VTY and connect to each configured
MGW instance on start up. This is in preparation for the
subscr_conn_fsm, which will allocate own MGCP connections while
processing BSSMAP assignment request and related messages.
It was decided to write OsmoBSCNAT with 3GPP AoIP support from scratch,
in a separate repository from OsmoBSC. The previous codebase was too
heavily tied into legacy SCCPlite code and had been removed from
osmo-bsc.git in Ia05dc76336a64a7f08962843b9a7cc19f2c83387.