VMware: Add in support for CPU shares in event of resource contention

The flavor extra specs for quotas has been extended to support
 quota:cpu_shares_level - the allocation level. This can be 'custom', 'high'
                          'normal' or 'low'.
 quota:cpu_shares_share - in the event that 'custom' is used, this is the
                          number of shares.

This enables the VMware driver to provide CPU shares.

DocImpact

Change-Id: Iba55f91a5374ec6a1412e726a7ae2fbf2641155b
Merge "VMware: Add in support for CPU shares in event of resource contention"