The variable storing start time of playbook run was named 't0'.
While perfecly fine from a functional standpoint,
and also in statistics and physics with subscript,
it is nonetheless a bit hard on the eye.
Tests were adjusted to reflect new var name.
Signed-off-by: Jiri Podivin <jpodivin@redhat.com>
Change-Id: Ie0796b1dcb8beacc08755fa04ff3cd4dd884ef59