1. An apparatus comprising: at least one processor; a memory coupled to the at least one processor; a first host group comprising a plurality of physical host computer systems; a first set of policies and a second set of policies for the first host group set by a system administrator that indicates how to place virtual machines on the plurality of physical host computer systems, wherein the first set of policies are initial placement policies that indicate how to initially place virtual machines on the plurality of physical host computer systems, and wherein the second set of policies are optimization placement policies that indicate how to place virtual machines on the plurality of physical host computer systems when optimizing the physical host computer systems; a graphical user interface that allows a system administrator to define the first set of policies and define the second set of policies for the first host group, wherein the graphical user interface allows the system administrator to set a first threshold for the initial placement policies and a second threshold for the optimization placement policies; a scheduler mechanism residing in the memory and executed by the at least one processor that places virtual machines on the physical host computer systems of the first host group according to the initial placement policies, monitors performance of the physical host computer systems of the first host group, and optimizes placement of the physical host computer systems of the first host group when the performance meets the second threshold of the optimization placement policies by migrating at least one of the virtual machines to a different one of the physical host computer systems; wherein the initial placement policies are simple policies for quickly determining an initial host for a virtual machine that are distinct from the more detailed optimization placement policies for migrating virtual machines to optimize placement.