Fix hostname regex pattern

Current hostname_pattern regex complexity grows exponentially
when given a string of just digits, which can be exploited to
cause neutron-server to freeze.

Closes-bug: #1378450
Closes-rally-bug: DE1514
Upstream-Review: https://review.openstack.org/#/c/135616/
  https://review.openstack.org/#/c/135623/
  https://review.openstack.org/#/c/135624/
Not-in-upstream: false