Fix in system_tests.sh help and settings.

- Fixed behaivour of the environment variable 'NODE_VOLUME_SIZE'.
- Added short description of '-V' option in the script ./utils/jenkins/system_tests.sh

Change-Id: I3fc92734ee5f19a7a31d95724ae30967dbcf4f92
(cherry picked from commit 7fdeaf4b4f041900bb8261db876ba333b99fe0c4)