CI: Add new tracks in cdash

As we are having more active branches, I feel that we will benefits of having a more organized dashboard in cdash

So far we have:

1. Nightly track, for master and scheduled (nightly) jobs
2. Experimental, for Merge-requests.

I propose the following:

1. master, for master builds (this is whenever we merge to master).
2. latest-master, same but not daily
3. release, for release builds.
4. merge-requests, for MRs.
5. Nightly, for daily schedule builds (everyday at 4 am as it is set now).

Note that I took the liberty to add a new variable named VTKM_CI_NIGHTLY which is exposed to nightly builds.