Various fixes and improvements

The following updates are included in this MR:

• Use configuration file defined in lb-nightly-configuration to connect to repository (#1 (closed)), including authentication (keytab for eos and (user, pass) for http)
• Use xrdfs stat to check if artifacts exist on eos instead of relying on eos mount
• Use subprocess.run instead of call (see https://docs.python.org/3/library/subprocess.html#older-high-level-api)
• Overwrite artifact while pushing to eos repository
• If URI of the repository in config does not have the scheme specified, always return FileRepository (even if it's /eos/some/path)
• Add tests for EosRepository