fixes #2260 Allow ERB in ENC global / class parameters

Adds code to parse the global & class parameters, and adds a setting (default true)
 to disable this if needed. Also has some tests.
Refactor Foreman::Renderer and HostTemplateHelpers into SafeRender

Tries to ensure all rendering of ERB goes through SafeRender, as well
as DRYing the foreman_url function. HostTemplateHelpers moved into SafeRender
as all methods are being called from there anyway.