Add library support for installing secrets.

Summary:
We install a bunch of secrets on our ec2 machines these days, and each
of them requires installation by hand at setup time.  I now provide 3
helper routines that handle the logistics of installing a secret, and
change all the setup.sh's to use them:

1) A routine that tells users to paste in a secret from phabricator
   passphrase, and writes it to the appropriate file.
2) A routine that tells users to paste in a secret from secrets.py,
   and writes it to the appropriate file.  This can either store
   the secret directly, or can copy the full 'var = "value"' line
   from secrets.py into the destination file.
3) A routine that tells users to paste in a password from phabricator
   passphrase, to allow us to decrypt a file in the aws-config repo
   that has the secrets in it.  This is used when there are a lot of
   secrets that need to be stored at once, and it's inconvenient to
   enter them all separately into passphrase.

For case (3), I also have added a default Makefile that makes it easy
to decrypt and encrypt these aws-config repo secrets.

In each case, the install command is a noop if the secret is already
installed.

A few fixes based on review.

Mostly comments, but a few contentful changes, including typos and an
improved makefile invocation.

I also standardized the use of nginx keys into a separate ssl/
directory.

Test Plan:
Fingers way crossed.  Next time we have to run some of these install
scripts, we'll see how things work!  I'm sure there are silly typos
and the like.

Fingers still crossed.

Reviewers: chris

Reviewed By: chris

Subscribers: emily

Differential Revision: https://phabricator.khanacademy.org/D15412