CP-6530: Add argument --config-dir to configure.py

Instead of using the first argument as the directory from which to fetch
specfiles, configure.py now looks for an argument of the form
"--config-dir=<config-dir>".

Signed-off-by: John Else <john.else@citrix.com>
CP-6530: Add argument --use-distfiles to configure.py

We can now configure whether to use the local distfiles cache.

Signed-off-by: John Else <john.else@citrix.com>