change default segdb url

set this to be something like

DEFAULT_SEGDB_URL = os.environ.get('DEFAULT_SEGMENT_SERVER', 'https://segments.ligo.org')

and then make sure we do the right thing when we extract the segdb_url from the INI (everywhere) to fall back to this.

This is only relevant for processes running at CIT, but it could/should provide better reliability by looking locally instead of authenticating with the remote server.