Switch default beaker nodeset to Debian
Ubuntu is still available but the default is Debian 7
Separate out server acceptance tests
passenger, thin, unicorn, and webrick can be tested independently
This implies differences to how tests are run, so documentation was
updated
The advantage here is no chance of bleed-over between test cases,
whereas the previous way required imperfect automated cleanup between
test cases.
This also permits testing one thing at a time to save a little bit of
time. Unfortunately, it also means that testing all cases requires
considerably longer because a new VM must be provisioned for each case.
Improve acceptance tests
Add an actual agent run to each test, to confirm that the master is able
to serve requests in some basic capacity. Would have caught the "unicorn
/ nginx support broken" issue where ssl headers weren't passed right.

Also moves some of the tests into common blocks to reduce duplication.
Merge pull request #123 from danieldreier/refactor_tests

Refactor acceptance tests