Rename "shared and specific runners" to "Instance and project runners"

Problem

This issue exists because we could not get those changes into https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18162

We need to solve the ambiguity of "shared" runners because we now also have group runners. Aside from that, we can be far more descriptive of specific and shared runners to rename them to the scope they actually are created in.

cc: @DylanGriffith @ayufan @bikebilly