Separate hypervisor application from virtual networking app

Currently, the virtual networking app (for example "Midolman" or "LinuxBridge") which is supposed to plug in neutron ports to the VMs and handle traffic flow throughout the virtual networking system is also acting as the VM creator and manager via the "HypervisorService" contract API (i.e. pure virtual interface) class.

These two tasks are not really related, as the VM creator/manager just has to create VMs (using KVM, or IP net namespaces, or docker, etc.), while the virtual networking app is basically plugging in neutron ports to actual interfaces on its data plane, and then managing traffic flow. It can use any VM type, as that's not relevant to its behavior.