Large RAN installations may benefit from distributing the RTP voice
stream load over multiple media gateways.
libosmo-mgcp-client supports MGW pooling since version 1.8.0 (more than
one year ago). OsmoBSC has already been making use of it since then (see
osmo-bsc.git 8d22e6870637ed6d392a8a77aeaebc51b23a8a50); lets use this
feature in osmo-hngw too.
This commit is also part of a series of patches cleaning up
libosmo-mgcp-client and slowly getting rid of the old non-mgw-pooled VTY
configuration, in order to keep only 1 way to configure
libosmo-mgcp-client through VTY.
Related: SYS#5091
Related: SYS#5987
Change-Id: I371dc773b58788ee21037dc25d77f556c89c6b61