Added --no-clean option for run_tests.sh

Added --no-clean option for run_tests.sh

Previously all the OpenStack resources (networks, VMs etc.)
created by user were deleted after executing tempest, vims
and rally test cases with run_tests.sh script.

New --no-clean option can be used for preserving resources
created by user so that the tests can be run also in setups
with deployed stacks.

Change-Id: I8c9505acf955c54b2b837d8ca916dc729c97bbca
Signed-off-by: Viktor Tikkanen <viktor.tikkanen@nokia.com>
(cherry picked from commit dce18f616c5c0bf019f72c68c872570f26d2978b)