(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