Fix ugliness in aurora help

"aurora help" currently has some aesthetic issues:

• in some places, the default name for a parameter is used: 'job status [--write-json] value'; metavars should be used to fill that in.

• In usage lists (like "aurora help job"), the long list of usage lines is difficult to follow; blank lines in between would make it clearer.

• In some cases, strings aren't getting handled properly, resulting in lists of characters isntead of strings (eg, "aurora help job create")