The example inventory file that 'jkastning' highlighted is confusing within this context. It doesn't show colocation at all AFAICT:
[mons]
MONITOR_NODE_NAME_1
MONITOR_NODE_NAME_2
MONITOR_NODE_NAME_3
[mgrs]
MONITOR_NODE_NAME_1
MONITOR_NODE_NAME_2
MONITOR_NODE_NAME_3
[osds]
OSD_NODE_NAME_1
OSD_NODE_NAME_2
OSD_NODE_NAME_3
[rgws]
RGW_NODE_NAME_1
RGW_NODE_NAME_2
It should be:
node1,node2,node3...node8
And you should have those nodes spread across the different roles so there is some colocation going on.