Remove pyudev dependency

pyudev was only used by the linuxbridge-agent to get the list
of virtual network devices. This can be got from /sys instead.
This patch fixes the problem where testr could not import the
lb-agent module because pyudev was not in requirements.txt.

Cherry-picked from dfa4112400173d2fc1c9bbe2bc8028d720fd7168

Change-Id: I0a78c91e97de4413f2ecf6fb56d2ff61b36baa78
Closes-Bug: 1269040
Closes-Bug: 1305428
Merge "Remove pyudev dependency" into stable/havana