Generalize and improve our utility function for parsing CLI args

At some point in time the clojure CLI library that we use to parse
stopped supporting a flag to indicate 'required' arguments.  Thus,
we were getting much farther into the code than we should have been
if you left off, e.g., the `--config` option to the `services`
program.  This also resulted in useless, unintuitive error messages.

Additionally, the function that we were providing in `utils` to
generalize our CLI argument handling for other PuppetDB command-line
tools included more arguments than would be applicable to some
tools.

This commit makes the `cli!` more general, and moves the app-specific
options back into the apps themselves.  It also adds support for
checking for `required` arguments and fails with a reasonable error
message if you are missing any.
Merge pull request #433 from cprice-puppet/maint/master/generalize-cli-validation

Generalize and improve our utility function for parsing CLI args