Added credential ID when using scitokens

[mambelli]

Added credential ID when using scitokens, to match glideins to requests by credential and to fix monitoring.

Improved also some token code, and removed stack trace when tokens are expired.

Improved comments and docstrings in frontend/glideinFrontendLib.py and frontend/glideinFrontendPlugins.py.

This PR fixes issue #242

[mambelli]

To fix a system with a monitoring problem you need to patch both the Frontend and the Factory running GlideinWMS v3.9.6 with 2 files from this PR. The other files in this PR can be ignored.

• Frontend: replace /usr/lib/python3.6/site-packages/glideinwms/frontend/glideinFrontendInterface.py with https://raw.githubusercontent.com/mambelli/glideinwms/v39/gh242_credential_id/frontend/glideinFrontendInterface.py
• Factory: replace /usr/lib/python3.6/site-packages/glideinwms/factory/glideFactoryEntry.py with https://raw.githubusercontent.com/mambelli/glideinwms/v39/gh242_credential_id/factory/glideFactoryEntry.py

After replacing the files restart Factory and Frontend.