Add skip funtion for checking enabled extensions in the l3 agent

This function check the extension configured in the l3 network agent
and if the extension is no enabled the test is skipped

That validation should not be applied when the l3 agent does not exist,
e.g.: OVN configurations

Change-Id: Icbc379d6b8ab0d720d2ae242d79949fb4dd9d27c
3 files changed