Check for the --config CLI arg before attempting to use it

Prior to this commit we tried to use the value of --config for
something, and then a few lines later we had the code that
would validate the presence of the parameter and fail with
a descriptive error message.  This was giving NPEs; this
commit just re-orders those steps.
Merge pull request #382 from cprice-puppet/bug/master/check-for-config-cli-arg-before-using-it

Check for the --config CLI arg before attempting to use it