Update comments in HostManager._get_instance_info
Update comments in HostManager._get_instance_info

The comments in this method are from when it was originally
added [1] in Kilo and the part about "not yet upgraded" is
way out of date (it was presumably just there for rolling
upgrades to Kilo) so that's all removed. In addition to
mentioning performance, I've also noted "isolation" as a
reason for disabling track_instance_changes in a split-MQ
cells v2 deployment [2].

[1] Ibfcd3b77dbde94db14a9e4027dbaceecb76182cd
[2] https://docs.openstack.org/nova/latest/user/cellsv2-layout.html

Change-Id: I28703dc7c106e9a00a31d4f84c103a7608b45de0

openstack/nova,mahak/nova,klmitch/nova,klmitch/nova,klmitch/nova,mahak/nova,openstack/nova,klmitch/nova,rahulunair/nova,rahulunair/nova,rahulunair/nova,openstack/nova,mahak/nova