In the gate, it is possible for /etc/hostname to not have a valid
SELinux security conext. So, reset them to default before we update
the hostname to avoid:

  TASK: [Assign hostname]
  failed: [node1] => {"failed": true}
  msg: Command failed rc=1, out=, err=Could not set property: Access
  denied

Closes-Bug: #1568047
Change-Id: Ie30c8437b310d1ea5bc75307375c255131dfe934
Needed-By: I2ff5fbb315832b8cf07c93d0dd0cdaacd958631e