1. An apparatus comprising: at least one processor; a memory coupled to the at least one processor; a collocation rule for a virtual machine group comprising a plurality of virtual machines; a plurality of situational enforcement policies for the collocation rule for a plurality of situations that indicate how to place the virtual machine group on physical host computer systems depending on the plurality of situations, wherein each of the plurality of situational enforcement polices indicate a situational policy enforcement type for a corresponding collocation rule and a corresponding situation; and a scheduler mechanism residing in the memory and executed by the at least one processor that places the plurality of virtual machines on the physical host computer systems based on the collocation rule and the plurality of situational enforcement policies for the plurality of situations.