Doc scheduler_host_subset_size option

fixes bug 1128545

Grizzly brings the scheduler_host_subset_size option, which gives
more than one host to be selected for scheduling, as opposed to
previously only the highest weighted host.

Adds the option to the config reference.

Missing <tr> tag fixed, spelling errors (not in the added row).

Change-Id: I4b96581ed7ef3d077b95a8e4aae39ba6f74cff53
Merge "Doc scheduler_host_subset_size option"