added abstraction in remote host and diagnostic service to prevent VS.Next dll from loading if remote host option is off.

the abstraction I added here is specific to diagnostic service feature. that abstract solely exist to prevent VS.Next dll from loading if remote host option is off.

this is different than IHostSpecificService abstraction I used to have that is generalized form of this abstraction which automatically figure out what to load based on option and layering/MEF composition of roslyn.

in this model, each feature need to take care of this kind of issue.