sysconfig mtu settings do not work correctly

This bug was originally filed in Launchpad as [LP: #1706973](https://bugs.launchpad.net/cloud-init/+bug/1706973)

Launchpad details

affected_projects = ['curtin']

assignee = None

assignee_name = None

date_closed = None

date_created = 2017-07-27T13:27:01.153459+00:00

date_fix_committed = None

date_fix_released = None

id = 1706973

importance = medium

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1706973

milestone = None

owner = smoser

owner_name = Scott Moser

private = False

status = confirmed

submitter = smoser

submitter_name = Scott Moser

tags = []

duplicates = []

_Launchpad user **Scott Moser(smoser)** wrote on 2017-07-27T13:27:01.153459+00:00_

Curtin's tests that derive from CentosTestNetworkMtuAbs (Centos66TestNetworkMtu and Centos70TestNetworkMtu) are currently not working.

That can be reproduced in curtin's vmtest with:
  ./tools/jenkins-runner -p4 tests/vmtests/test_metwork_mtu

or more directly
  ./tools/jenkins-runner \
     tests/vmtests/test_network_mtu.py:Centos66TestNetworkMtu \
     tests/vmtests/test_network_mtu.py:Centos70TestNetworkMtu