Support grabbing request logs for more than one backend type. We're interested in getting logs for our high memory backends as well as our mapreduce backends, as these are used for publish.

Test Plan:
Create a file called "status" w/ the following contents:
    No logs for version [default], trying version 0830-55a794207f35
    appengine_versions: 0830-55a794207f35
    11775 request lines found
    [2013-09-04 01:25:23.208695] Fetching logs from [2013-09-04 00:58:40, 2013-09-04 00:58:50)...
    appengine_versions: 0903-3de889ad93da
    12294 request lines found

Before applying this diff, run:
    $ python src/fetch_logs.py --backend --file_for_alternate_appengine_versions=status

And verify that output references two different mapreducebackend-suffixed versions:
    Looking at these appengine versions: ['0830-55a794207f35-mapreducebackend', '0903-3de889ad93da-mapreducebackend']

After applying diff, run same command and verify that output references two mapreducebackend-sufixed versions and two highmembackend-suffixed versions:
    Looking at these appengine versions: ['0830-55a794207f35-mapreducebackend', '0903-3de889ad93da-mapreducebackend', '0830-55a794207f35-highmembackend', '0903-3de889ad93da-highmembackend']

**Note** I have not run this entire script to completion nor have I run it on our live analytics machine, the above test is all I've done.

Reviewers: jace, eliana

Differential Revision: http://phabricator.khanacademy.org/D3979