make MGW endpoint domain name part configurable
So far we create endpoints at the MGW using names like "1@mgw", i.e. with domain name "mgw".
When connecting to an SCCPlite MSC, I find the MSC issuing a CRCX on endpoint "email@example.com" (the IP address osmo-mgw is bound to).
The domain name used by MSC and osmo-bsc must be the same, so that they address the same endpoint, and osmo-mgw expects exactly one given domain name.
osmo-mgw's domain name is configurable by 'mgcp' / 'domain NAME'.
Add to osmo-bsc an 'msc 0' / 'mgw domain NAME' config item, so that it can be set to match the domain name the SCCPlite MSC sends.