Augment update_phabricator_repositories to get github private repos.

Summary:
I do this by using an access token for the role user, who has
privileges necessary to see private repos.

Use the necessary ssh format for private repos.

Phabricator can use an auth key if needed, for private git repos, but
we need to tell it to use the ssh format.  Now we do.

This lets us get rid of the previously hard-coded private repos.

Test Plan:
I copied github_role_account_repo_token from secrets.py to
~/github.repo_token and then ran
   internal-webserver% ./update_phabricator_repositories.py -n /tmp
and saw it say:
   Adding 27 new repositories to phabricator
   Adding new repository JSContextBenchmarking: url=https://github.com/Khan/JSContextBenchmarking, callsign=GJSC, vcs=git
   Adding new repository analytics-private: url=https://github.com/Khan/analytics-private, callsign=GAP, vcs=git
   Adding new repository blocks-playground: url=https://github.com/Khan/blocks-playground, callsign=GBP, vcs=git
   Adding new repository business-cards: url=https://github.com/Khan/business-cards, callsign=GBC, vcs=git
   Adding new repository content-tools-tools: url=https://github.com/Khan/content-tools-tools, callsign=GCTT, vcs=git
   [etc]

These are all private repos that we weren't covering previously.

Ran
   ./update_phabricator_repositories.py -n /tmp
and it said things like:
   Would add new repository khan-report-deprecated: url=git@github.com:Khan/khan-report-deprecated, callsign=GKRD, vcs=git PRIVATE

It did not say to remove any of the hard-coded private repos.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13115