Validate the owner type. This will fix the problem where the API returns 500 with a malformed request that contains a wrong owner_type

Validate owner type on user model to return 422

Use validates_inclusion_of to validate owner type in Host::Base

Blank owner type is not a valid value

Use OWNER_TYPES and allow empty owner_type for hosts other than Host::Managed

Added test for empty owner_type when creating unmanaged host (should pass)