Delete tag via API

### What does this MR do?

Implements deleting a tag via the API.

### Are there points in the code the reviewer needs to double check?

On success, it returns the name of the deleted tag. This is similar to the [delete branch API](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/branches.rb#L111).

###  What are the relevant issue numbers / Feature requests?

* This MR closes #1575 
* Closes http://feedback.gitlab.com/forums/176466-general/suggestions/6599203-delete-tags-through-the-api

@stanhu Could you do a review?

See merge request !1862