Make fetch directory configurable

Make fetch directory configurable

Currently, the fetch directory is created in your working directory
(where ansible is run from).  We prefer to not keep any state in this
directory and would prefer to have the fetch directory configurable so
we can store it outside of our code checkout.

This commit creates a new variable in each role called
`fetch_directory` (defaulting to the previous value of 'fetch/'), and
then updates each reference to 'fetch' to use the new variable instead.

Closes issue #383