(PUP-405) Remove cleanup of rails based storedconfigs

This removes cleaning of rails / active record storedconfigs from
the node "clean support"
(PUP-405) Remove catalog active record indirection
(PUP-405) Remove active record indirection for facts
(PUP-405) Remove active record indirection for node
(PUP-405) Remove active record indirection from resource
(PUP-405) Remove active record from the indirector
(PUP-405) Remove unused require of rails in compiler spec

There was a require of rails, and then it was stubbed out.
(PUP-405) Remove rails related settings
(PUP-405) Remove all things rails related

This removes all implementation and specs related to rails.
Basically rails support in puppet lived in its own bubble.

This also removes the rails feature, and support for setting up a
database for tests.
(PUP-405) Cleanup remaining settings related to storeconfigs and utils

This modifies settings:
* thin_storeconfigs is dropped
* puppetdb is made the default backend
* descriptions updated to not mention active record based things
* tests updated to reflect changes

This commit also includes
* utility methods clean up (now unused method)
* small maint tweaks
(PUP-405) Remove acceptance test that requires active_record

This removes the acceptance test "ENC node information is used when
store configs enabled (#16698)" since it tests the combination of
an ENC and the use of active record based store configs.

A corresponding test should be added to test a real supported
combination of services (node classifier / puppet db), but this cannot
be done in the puppet code base / acceptance suite. See PUP-3707 for
follow up.
(PUP-405) Remove dead code from face/node/clean.rb

The clean action contained the notion of "unexport" which was only
supported for rails/active record based store configs.
(PUP-405) Remove now unused active record/sqlite from Gemfile
(PUP-405) Rephrase the description of the clean action
(PUP-405) Clarify the description of what the clean action removes
(PUP-405) Remove jdbc-sqlite3 from Gemfile as it is not needed
(PUP-405) Remove ext/nagios/naggen as it is active record specific
(PUP-405) Remove active-record specific test in puppet-test
(PUP-405) Remove rails from debian control file
(PUP-405) Remove thin_storeconfigs setting

This setting reappeard as a result of rebasing due to nearby
changes. Once again removed.