(PUP-3334) Clear environment settings caches

Prior to this commit, environment settings were not being updated
when environment.conf was changed, even if environment timeout had
expired. This was because they were being cached in places in
Puppet.settings that were not being cleared.

Now, all the cached data for a particular environment is cleared
when the environment timeout expires.
(PUP-3334) Add test to ensure cache is cleared

In order to ensure that we don't encounter this issue again, add
a test case to environments_spec to ensure that the cache is being
properly cleared.

Additionally, fix a small bug in environment's list method where
a string should been converted to a symbol.
Merge pull request #3197 from hkenney/issue/stable/pup-3334_fix_environment_conf_not_changing_issue

(PUP-3334) Fix issue around settings not updating with environment.conf