Auto Completion: Attribute Keys for Update & Create (Simple)

[steve-r-west]

Let's support auto completion of attribute keys for update and create, so when you type epcc create customer X you can get auto complete values for the X.

Acceptance Criteria

1. Support for completion of attributes for update and create. So when you are in the Key/Value part, and the next arg is a key, we should support completion of the key.
2. We should filter out already typed keys in the command line.

Out of scope : Don't worry about array parameters right now, we will fix that in a different ticket.