Fix InsecureRequestWarning for Python API

[ParthS007]

Reported by @lukasheinrich

/Users/lukasheinrich/Code/pmssm/recastenv/lib/python3.7/site-packages/urllib3/connectionpool.py:988: 

InsecureRequestWarning: Unverified HTTPS request is being made to host 'reana.cern.ch'. Adding certificate verification is strongly advised. 

See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings

[tiborsimko]

@lukasheinrich We do have proper SSL certificate for reana.cern.ch, so I guess this can be fixed now easily... It seems to be an issue coming from inside the pmssm codebase?