Clean up and document AMI construction

[axch]

As I understand it, this is some shell scripts and a Makefile in the ami directory. As I further understand it, the Makefile also contains some cruft pertaining to starting instances that is now redundant with the cloudformation setup in the aws directory.

Success conditions for this ticket:

• The ami directory doesn't have any excess cruft
• There is an English guide to how to build a fresh AMI using the contents of the ami directory

[riastradh-probcomp]

Not really sure it's important to remove the test targets -- they may make it easier to, well, test a new AMI before deploying it with CloudFormation.

[axch]

OK, great. Then the scope changes to document that use case.