Pass project_config_repo from single_use_slave

Pass the project_config_repo variable from single_use_slave.pp
manifest to the nested slave_common.pp to be able to set it's
value from parent scripts (e.g. prepare_node.sh)

Change-Id: I3c765a72fd48624cddc6146feb94331b2c764df3
Merge "Pass project_config_repo from single_use_slave"