The PowerVM patch to implement the update_provider_tree method (in a
fashion backward compatible with the existing get_available_resource
method) [1] was in flight at the same time as a
bp reshape-provider-tree patch introducing the 'allocations' kwarg [2]
and another patch which provided a ComputeDriver helper method for
calculating the reserved value for the DISK_GB resource from config [3].
This brings PowerVM's update_provider_tree method up to scratch with
those changes.
[1] I96da858e6a63e3339a009fd17ee0d8939b364510
[2] Ic062446e5c620c89aec3065b34bcdc6bf5966275
[3] I72c83a95dabd581998470edb9543079acb6536a5
Change-Id: I89ad36f19672368a1f795e1f29c5af6368ccfeec
a148b11410