fixes #2405 - disable DNS orchestration during TFTP tests, causing timeouts
refs #2254 - STI Host relationships
fixes #2263 search Hostgroup by label
fixes #2247 Provides an option to disable the use of gravatara

A new setting is added, use_gravatar. It defaults to true. If it is set to false, then the gravatar
call out is not used and only the default user icon is used.
fixes #2407 - Add STI to settings model

Also adds some tests for rendering new/missing STI models, and
refactors the old default_settings loader into the new sub-models.