fix a misleading docstring

GroupAntiAffinityFilter was introduced to filter a set of hosts,
howerver the docstring saying filter a set of instances which need to be
fixed.

Change-Id: Icf66a3ad15b4135c06e2e2ccc62837a346fcfdf7
Merge "fix a misleading docstring"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp