Multiple Gateways[5/2]: sourceRoute handles exceptions

sourceRoute.configure and remove now handle IPRoute2Error
exceptions that can be thrown if using DynamicSourceRoute
and the Iproute2 configurator. The Iproute2 configurator
calls 'ip rule add/remove, ip route add/remove' that can
throw exceptions if the configuration fails.

For example:
  1) If we try to add routes if the interface is down
     (Which might happen during host bootup)
  2) If we try to configure routes that already exist
  3) If we try to remove rules that don't exist

Patch Sets 2-4:
* Rebases

Patch Set 5:
* Moved exception handling to sourceRoute.py

Patch Set 6:
* Removed unused imports

Patch Set 7:
* Updated commit message

Change-Id: Ib550a63257de28df1c44a92aeb0dc078f2edd906
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/16629
Reviewed-by: Dan Kenigsberg <danken@redhat.com>