Use '=' as separator in Linuxbridge configuration

- Replace ':' with '=' as key-value separator for
  physical_interface_mappings entry in the Linuxbridge configuration
  example file.

- See comment by darragh-oreilly at https://ask.openstack.org/en/question/5125

Change-Id: I8ccd1bcc62f20ac82702c902c85efd62b3aa3344
Merge "Use '=' as separator in Linuxbridge configuration"