Fix envutils.get_global (prev. default_deployment_id) errors management

* Since envutils.get_global doesn't raise anymore 'InvalidArgumentsException'
(due to this fix https://launchpad.net/rally/+bug/1291298) we are encountering
issues when get_global simply returns None. That's why the bool parameter
doRaise was added to get_global, so when needed the method will raise
'InvalidArgumentsException' and in the case of #1291298 get_global won't raise
any exception and will simply return None.
* Fixed unit test of Tempest Install command, deploy_id was always None in the
unit test and once the above modification applied, an InvalidArgumentsException
was raised which caused the failing of the test.

Change-Id: I18e2ac99e6412c3ae67769390e390d4210243381
Closes-Bug: #1293454
Merge "Fix envutils.get_global (prev. default_deployment_id) errors management"