Update SL7 installation script

The SL7 installation script needs to have a few sections updated, to do some general things (start using dqsegdb-tools and dqsegdb-private repos) and some specific things (add SciTokens code that works on P3).

This ticket should have new items added to the checklist, discussed in comments, and checked off in the checklist when they're done.

Some things to be done:

• Handle Python 3 version of Python server
• Install Python 3 version of SciTokens code
• Move available files from local filesystems to dqsegdb-tools (partially done)
• Move available files from local filesystems to dqsegdb-private (partially done)
• Remove code related to user ldbd after the transition to user segdb is done (or at least far enough along that the code is no longer needed)
• Modify crontab backup system to use user segdb's home dir, then pull the latest one from there (esp. in run_block_8)