Adding support for multiple hypervisor versions
Adding support for multiple hypervisor versions

This is to prevent instances with newer tools from being provisioned
on older hypervisors.
Added comparison of hypervisor version from the compute-node and image
metadata in the image properties filter.
In host scheduler image_prop_filter, the 'hypervisor_version_requires'
image properties, if available, is compared to the host_state's
hypervisor_version, if available.
hypervisor_version_requires should be added on the image as metadata and
should have a operator and version value.
Examples : ">=6.0", ">6.0, <6.2", ""!=6.1"

Partially Implements blueprint xen-support-for-hypervisor-versions
Cell scheduler changes coming up in another commit.
DocImpact

Change-Id: I6fbf232adf9ac78ebb8ac2985739f163f4517d14

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects