Generalize bot's separate endpoints support

Description

Factorize the buildbox bot code and mechanism for using separate end-points for remote execution and CAS server.

Changes proposed in this merge request:

• Declare new bgd bot CLI options for CAS server location and TLS keys.
• Clean-up the bgd bot buildbox CLI options.
• Port the temp-directory bot to the new separation.
• Adapt the buildbox bot to use the common mechanism.

This merge request, when approved, will close issue/bug:

#68 (closed)

This merge request depends on:

!49 (merged)