Update compute disk I/O pollster to use cache

Use the pollster cache to store the disk statistics
for an instance. Eventually the single pollster will
be broken up into separate pollsters that all use
the cache.

blueprint one-meter-per-plugin

Change-Id: I8aa8879e00902a2ba9f78104ee566a42effcb999
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge "Update compute disk I/O pollster to use cache"