add hostname_method as a attribute

Expose hostname_method as an attribute and in diamond.conf.erb
This is the default according to the configuration file.

https://github.com/BrightcoveOS/Diamond/blob/a15c4c178d9cc0c1c111652a8a6323206a9f31ac/conf/diamond.conf.example#L144-L160

If someone overrides the value with `nil` the configuration file will have a line with no value. I do not know how Diamond would react.

If you like I can make the line only written if the attribute is not nil.