mgw: add constants for mgw endpoint and domain name

The testcase for osmo-mgw hardcodes the mgw domain and also lacks
an endpoint identifier, which was deprecated recently.

- replace the hardcoded occurrences of the mgw domain name with
  a constant

- add a constant for the rtpbridge endpoint identifier

- add a testcase that still uses the old method without prefix
  on the endpoint identifier.

Change-Id: If4455c4cb521270b2fe24881ade9b578a5132205
2 files changed