Add fqdn_external

Add fqdn_external

In newton, we used to construct the fqdn_$NETWORK in puppet-tripleo for
external, internal_api, storage, storage_mgmt, tenant, management, and
ctrlplane. When this was moved into THT, we accidently dropped external
which leads to deployment failures if a service is moved to the external
network and the configuration consumes the fqdn_external hiera key.
Specifically this is reproduced if the MysqlNetwork is switch to to
exernal, then the deployment fails because the bind address which is set
to use fqdn_external is blank.

Change-Id: I01ad0c14cb3dc38aad7528345c928b86628433c1
Closes-Bug: #1697722