Code review comment for lp:~gmb/maas-test/fix-up-proxy-config-dir-for-tests

Looks good, but I'm concerned about punting everything to the user.

[1]

+    # maas-test config
+    parser.add_argument(
+        '--config-dir', type=text_type,
+        default="~/.maas-test",
+        help="The directory in which to store maas-test config and cache "
+        "files. Defaults to %(default)s")

I think we should use the xdg stuff I mentioned to rvb yesterday, and
not have yet another configuration option. Or is there a compelling
use-case for allowing this? This feels like getting the user to choose
because we can't make a decision.