Path concatenation without forward slash

This is reported by Manjunath from Slack channel. For work_dir: "dbfs:/feathr_getting_started" The user was running jupyter notebook from windows. without forward slash('dbfs:/feathr_getting_started') the job configs were coming as '--join-config', 'dbfs:/feathr_getting_started\feature_join.conf'

To fix this, the user need to add foward slash: work_dir: "dbfs:/feathr_getting_started"

We should add validation or automatically append a forward slash. The best way would be using Path utils that can join files/folders properly.