Have L3 agent catch the correct exception

L3 agent imports the processutils module to catch exceptions that
wouldn't ever be thrown because the underlying execute() being
called is the one from neutron.agent.linux.utils which raises a
RuntimeError on failure.

Also, processutils is now part of oslo.concurrency. So when we
actually start using it, we'll use it from there.

Closes-Bug: 1401626
Change-Id: I43874e1b63a0ba7b01415cafe0538f4343057066
Merge "Have L3 agent catch the correct exception"