Nits and stuff

[webbnh]

This PR consists of a bunch of tiny fixes of minimal consequence which I've gradually accumulated over the last month. They were things which I ran into in the course of making other changes, but, since they had nothing to do with the changes I was making, I kept them on a side branch. At this point, there are enough of them to warrant putting up a review.

• Remove unused definitions from the Server container build and from load_keycloak.sh
• Correct a docstring
• Correct the import of the urllib3 package and add a requirement for it: the package is apparently included inside the requests package, and we were referencing it there instead of declaring our own dependency on it.
• Add defaults to the deploy-dependencies script: this script is normally invoked by a higher-level script which provides these definitions; if a user wants to run this script independently, then s/he has to "just know" to provide these definitions in the environment; this change provides reasonable default values if the variables are not already defined.
• Try to report errors in the containerized Agent command invocation. The containerized Agent container images are intended to be invoked with a shell command (typically a Pbench Agent script) to run. If this is omitted, or if the container invocation command line is formed incorrectly with command options positioned after the image spec, then the exec call inside the script which is run as the container entrypoint will fail, and this results in a cryptic response (if any) for the user. This PR modifies the entry point script script to try to handle these cases better for the user.

[dbutenhof]

There's a shell linter? And you're running it? Weird. 🍤

[webbnh]

There's a shell linter? And you're running it? Weird. 🍤

Yeah, there is. Yeah, I am. And, yeah, it makes life "interesting". I don't necessarily agree with everything it says, but there are enough pitfalls in shell programming that I do appreciate having the heads-up when I'm doing something questionable (or when there is a better/alternate way to do it that I wasn't aware of).