allow specifying base path that is subtracted from abspath in upload_to_server

[lukasheinrich]

partially related to #491

upload_to_server does some calculation on what the final path should be by substractiing the current working directory.
This hampers the ability to submit workflows form a different directory

It would be great to be able to specify the "file root" in upload_to_server (which can very well default to os.getcwd())

reana-client/reana_client/api/client.py

Line 545 in 03460b7

save_path = path.replace(os.getcwd(), "")