Cell Scheduler support for hypervisor versions

Adding the hypervisor version to cell capacities
Creating a filter in cell scheduler to filter cells based on
prominent hypervisor version

Partially Implements blueprint xen-support-for-hypervisor-versions

DocImpact: Adding a filter in cell scheduler to pick cells based on
the hypervisor version required by the image. The image can have
metadata called hypervisor_version_requires which will be matched
against the cell's prominent hypervisor version.
The prominent_hypervisor_version is stored in nova.conf under cell
capabilities.

Change-Id: Ied716a5964fd86a919ffb8bac99100afb770896e
Merge "Cell Scheduler support for hypervisor versions"