add tag for triggering pod restart tests on gerrit patch sets

Introduce new 'trigger-string' in ci-management/jjb to start "Voltha Pod Restart" test cases for any gerrit patch sets that find running these tests useful to validate their change set. The 'trigger-string' can be pasted in the comments section in the gerrit patch and the test will start automatically. Refer to 'hardware test' or 'functional tests' for example on how to achieve this in ci-management repo.