DOC-1281: Puppet Server gems, puppetserver_vs_passenger

Some edits to gems and puppetserver_vs_passenger pages.
(maint) rename functions for clarity + consistency

Rename the three primary functions that return ring handlers to 'build-ring-handler'.
(SERVER-114) endpoint to flush environment cache

Add a new HTTP endpoint to the admin API, /v1/flush-environment-cache,
which tells the Puppet ruby code to invalidate all of its environment
caches.
(SERVER-114) make API more RESTful

Change the endpoint which flushes Puppet's environment cache to be more
RESTful.  Previously, it was a POST to /flush-environment-cache.  This
commit changes it to a DELETE to /environment-cache.
(SERVER-114) add comment about test coverage
(MAINT) Use GEM_SOURCE env var in Gemfile
Merge pull request #297 from waynr/maint

(MAINT) Use GEM_SOURCE env var in Gemfile
(SERVER-114) specify puppet.conf used by test

This allows the test to pass when the default certname is not
'localhost'.
(SERVER-114) /environment-cache header handling

Make the implementation of /environment-cache tolerate an 'Accept: */*'
header in the request.
Merge pull request #294 from KevinCorcoran/SERVER-114/add-flush-env-cache

SERVER-114 - add endpoint to flush env cache
(SERVER-208) Improve error message for missing `puppet-admin` section

This commit improves the error message for a missing `puppet-admin`
section in the Trapperkeeper configuration at startup.
Merge pull request #298 from camlow325/maint/master/SERVER-208-improve-missing-puppet-admin-config-message

(SERVER-208) Improve error message for missing `puppet-admin` section
(maint) add config for admin API
Merge pull request #299 from KevinCorcoran/maint/update-config-for-admin-api

(maint) add config for admin API
(MAINT) Ensure gem subcommand tests honor GEM_SOURCE
Merge pull request #300 from waynr/maint

(MAINT) Ensure gem subcommand tests honor GEM_SOURCE
(maint) docs for admin API + /environment-cache

* Add docs on the new /environment-cache endpoint.

* Update the config doc with more detail about configuring the admin
  API.

* Minor formatting / wording fixes in the /certificate_status section.
(MAINT) Update developer docs re: running from source
(MAINT) Fix typos, simplify git submodule init for quickstart
(MAINT) Add some notes about debugging / ruby