Add a config item external_hostname

This is the reported DB hostname when listing database entities through
the REST interface.

This way, db_host can be set to localhost, and it can report the created
database's host as something reachable from the network
Merge pull request #48 from brummett/external-name

Add a config item external_hostname
Merge branch 'master' into create-db-refactor

Conflicts:
	lib/TestDbServer/Configuration.pm
	test_db_server.testing.conf