Update the pbench_demo script to add --server

[webbnh]

This PR is a follow-on to #3494, which removed the use of PB_AGENT_SERVER_LOC from the containerized Agent pbench wrapper script (it turns out that the demo was depending on it...). Now that we have the --server option on pbench-results-move, we use that in the demo script (instead of dynamically creating an Agent config file) to specify the server location.

While making the above addition, I also dusted off the demo file:

• I reworked the header comment to clarify the purpose of the file (i.e., it's more documentation than tool) and to explicitly list the required input environment variables.
• I removed the explicit override of the container image -- that's a feature of the pbench script, and if the user wants it s/he can define it in the environment when running the demo -- and reworked the comment which mentions it.
• I removed the feature of specifying the API Key token as a command line argument -- I'm not aware of anyone using it, and it therefore imposes useless complication. Providing it via the environment is adequate (and can be done as part of the command line anyway, so the difference is just syntactic sugar).
• The script now makes sure during setup that the Server location and API Key environment variables are defined and that the Server location is properly formatted, so that there are fewer surprises after the workload run is complete when we try to upload the result.
• I repositioned the code which creates the FIO work directory and added an explanatory comment for it.
• I changed the script execution to echo only the "interesting" commands, instead of the whole script including the setup.

NOTE: since @pravins wants this change for use over the weekend, and since this is a change to the contrib area, and since there aren't any reviewers around today, I'm going to go ahead and merge this once the tests pass. If someone has comments, we can iterate on them.