netinfo: Do not mix local time and UTC, improve tests

dhclient is run in the background so the functional test can
finish. Both date-time formats dhclient uses are now tested.

In the non-functional test, leases are created dynamically.

Change-Id: I1b2a33209b45001c427d5712d57326394cbeb9d0
Bug-Url: https://bugzilla.redhat.com/987813
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24192
Reviewed-by: Dan Kenigsberg <danken@redhat.com>