If `--pull` or `-p` argument is passed to blucon, it will now run a
`docker pull` command prior to launching the testcases with `docker
run`.
This should prevent the testcases from using stale docker images if
already present on the machine.
JIRA: VAL-107
Change-Id: Id4b7947a0de7abf07f69812c6deef71919b9041a Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>