CLOUDSTACK-7641: Do not always ask libvirt to refresh a storage pool

On larger (especially RBD) storage pools this can take a lot of
time slowing operations like creating volumes down.

The getStorageStats command will still ask a pool to be refreshed so
that the management server has accurate information about the storage pools.

On larger deployments, with thousands of volumes in one pool, this should
significantly improve storage related operations
CLOUDSTACK-7583: Send VmStats to Graphite host when configured

This allows external processing of VmStats information without using
the usage server of CloudStack

Statistics are being send to Graphite using UDP and not TCP.

UDP is used to prevent the management server waiting for TCP timeouts
when the Graphite server is unavailable