Fix the 'MTU' option in networkTests

The 'mtu' option should be in lower case. It doesn't fail with ifcfg
configurator, because the 'MTU=1234' passes through to the ifcfg file
as an extra option.

Change-Id: Ic09a5dd6e8f760eda5dece19bc7e0da2e49611e0
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/18409
Reviewed-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>