Update compute vnic pollster to use cache

Use the pollster cache to store the VNIC info
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: I8d9af0006d280f26ea768db97608074c5cfc9457
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge "Update compute vnic pollster to use cache"