Fix GAE SDK import in gae_dashboard_curl.py

Summary:
On the analytics machine, a PYPI module has been installed that provides
"google", but has no "appengine.tools" submodule. Other scripts had
already migrated to using gae_util.py's SDK lookup and were not
impacted, but this fetch_usage.sh (and other scripts depending on
gae_dashboard_curl.py) broke with the error:

Traceback (most recent call last):
  File "/home/analytics/analytics/src/gae_dashboard/gae_dashboard_curl.py", line 20, in <module>
    from google.appengine.tools import appengine_rpc
ImportError: No module named appengine.tools

I also updated the gae_dashboard/fetch_* scripts to be parameterizable,
it made local testing easier.

Test Plan:
$ cd analytics/src/gae_dashboard
$ srcdir=. ./fetch_usage.sh
Importing usage reports newer than 2014-04-01
Importing 0 documents

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D7951