Implemented silent mode for gerrit review

Implemented silent mode for gerrit review

Added silent_mode parameter to Gerrit constructor. If silent_mode is
True gerrit review command is not called and review mark is not reported
back to Gerrit.

job_submitobs gets value of silent mode from GERRIT_SILENT_MODE
environment variable. This variable must be set in jobs/configuration
file for Jenkins instances, where reporting back to Gerrit is
prohibited.

Change-Id: I78a57885d3619eee01f48227bb6a299d3b1c8a06
Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com>
Reviewed-on: https://otctools.jf.intel.com/review/5651
Tested-by: OTC Tools Tester <ed.bartosh@linux.intel.com>
Reviewed-by: Hasan Wan <hasan.wan@intel.com>