Check neutron agent extension support in Info page

Some neutron plugin don't support agent extension, and
Horizon show 404 error on admin info page.
In this commit, we are going to add check if agent
extension supported or not.
Closes-bug: 1331032

Change-Id: I5cd3ec747c650fc2b826d55f39be618eca1b7caa
Merge "Check neutron agent extension support in Info page"