network/api.py: delete network's orphans + test

When addNetwork fails on ifup, it leaves orphan devices.

setupNetworks function is changed to remove leftovers in case of
addNetwork's ConfigNetworkError with code ERR_FAILED_IFUP.

testHonorBlockingDhcp tests for bridge leftovers.
testNetworksEmergencyDevicesCleanup tests for leftovers of more
complex network which overwrites existing devices.

Two/four added tests takes almost five minutes so they (and
testHonorBlockingDhcp) are marked as @slowtest.

Change-Id: I3ef7ab74a53a8c2bd49710c01771b623c41cbf88
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31336
Reviewed-by: Dan Kenigsberg <danken@redhat.com>