Make ip rule comparison more robust

Make ip rule comparison more robust

I found that ip rules would be added multiple times in new address
scopes code because the _exists method was unable to reliably
determine if the rule already existed.  This commit improves this by
more robustly canonicalizing what it reads from the ip rule command so
that like rules always compare the same.

Change-Id: I6d0c208f0ed8e65cdb750789321a7ad6ca1b77c2
Partially-Implements: blueprint address-scopes