Add tag count to `/hosts` endpoint

[karelhala]

Since we allow tags to be assigned to systems now we should also allow for fetching them directly when listing all hosts. It doesn't matter if we are going to show only tagCount or full tags associated with host.

[ryandillinfelton]

The PR above adds the tag count to the default response from /hosts. I don't know where we stand on actually adding it, but it was a quick change so I figured it couldn't hurt to have it as an option.