Usability issues when registering an agent via command line

• Fix the stork-agent --help usage text to show /etc/stork/agent.env as default for env-file.

        --env-file=                                 Environment file location; applicable only
                                                    if the use-env-file is provided (default:
                                                    /etc/stork/server.env)

• stork-agent and stork-server should use the default env files as the help usage text suggests.

  This should apply to stork-agent register as well. It should be possible to run it simply like this:

  $ stork-agent register
  FATA[2024-06-19 11:55:35]             main.go:197   Problem parsing agent host: address localhost: missing port in address 

  Currently, env files are not used unless explicitly pointed towards.

  I suspect the intention was to describe the behavior of stork daemons when running under service managers, but in that case, the help displayed by the binary shouldn't say there is a default.

• Fix the ARM example under this section saying that you can register by just specifying the -u flag. If the previous point of allowing to register without any flags is implemented, the -u should probably just be removed.

  This is not the case as you can see. The same error is displayed rather trying to suggest that -a is missing:

  $ stork-agent register -u http://127.0.0.1:8080
  FATA[2024-06-19 11:53:40]             main.go:197   Problem parsing agent host: address localhost: missing port in address

• Make the values accepted by -a and -u consistent in format.

  Adding a -a with the same value as the -u as shown in the ARM presents a new error:

  $ stork-agent register -a http://127.0.0.1:8888 -u http://127.0.0.1:8080
  FATA[2024-06-19 11:53:47]             main.go:197   Problem parsing agent host: address http://127.0.0.1:8888: too many colons in address

  Through trial and error, you are able to determine that -a does not like the protocl part. This one works:

  $ stork-agent register -a 127.0.0.1:8888 -u http://127.0.0.1:8080

  But if you try to remove the protocol part from -u, it fails again.

  $ stork-agent register -a 127.0.0.1:8888 -u 127.0.0.1:8080
  ERRO[2024-06-19 11:54:25]         register.go:376   Cannot parse server URL: 127.0.0.1:8080       error="parse \"127.0.0.1:8080\": first path segment in URL cannot contain colon"
  FATA[2024-06-19 11:54:25]             main.go:217   Registration failed 

• Remove -a and -u and replace them with the already existing flags --host, --port, --server-url which are not used. This makes the previous point.

  In fact, --server-url is displayed twice in the help usage text: once under general info and once under register-specific commands.

  The command would then look like this:

  $ stork-agent register --host=127.0.0.1 --port=8080 --server-url 127.0.0.1:8080

• Have a way to stork-agent register non-interactively.

  This appeared as a requirement in the check-pkgs job, but may also be useful for someone who may want to automate registration. While registration is possible without a token, the command line requires interactive input under >>>> Server access token (optional): so a script would just block. You cannot bypass it with piping or redirecting a newline to it because you are met with an inappropriate ioctl device error. Using -t with an empty value doesn't work either.