vmtest: fix artful networking
- Remove skip_by_date calls in vmtests that are no longer valid
- ifenslave brings in ifupdown, so filter out ifenslave package if target
release is artful (LP: #1706744)
- vlan and bridge-utils are not needed in artful, filter those packages from
being installed in artful as well.
- Adjust artful bonding test to skip checking for ifslave and instead check
that ifenslave is *not* installed
- Drop mtu settings in network-static-routes configuration, the test case
only needs to test static routes
- Move static routes under an interface subnet to be compatible with
netplan format which requires routes under an interface
- Refactor ip_route_show parsing, ignoring default route and fetching
variable settings like 'proto'.
- Skiptest bridging in artful, need a cloud-init fix for stp in netplan
(LP: #1721157)
- Skiptest for mtu in artful, need networkd to support mtu6 support
(LP: #1671951)
Fixes: LP:#1714028 LP:#1718216, LP:#1706744