Having the zaza test configuration charm_name = mysql-router adds
mysql-router as an application that is never actually instantiated. This
leads to zaza failures looking for a unit of that application.

Simply change the charm_name to an instantiation of mysql-router
resolves this. i.e. keystone-mysql-router which is in each bundle.

Change-Id: Ib89d11352635e077f062b3f86a4024802f59419f
Closes-Bug: #1931117