generator: sanitize values ending with the fqdn

Some options (neutron_id in the neutron bigswitch plugin) use the
hostname/fqdn to generate default values prefixed with a static string.
This patch handles such options and sanitize the default value by
replacing the hostname/fqdn with the 'oslo' string.

Closes-Bug: #1298589
Change-Id: I2471d638f42fc425efc928b280e1b47c23fddabd
Merge "generator: sanitize values ending with the fqdn"