VMWare Driver - Ignore datastore in maintenance mode

A datastore can be part of a datastore cluster. Any
datastore which is part of a cluster can be put in maintenance
mode. Instances cannot reside on a datastore if it is in
maintenance mode.
This fix ignores a datastore in the following scenarios
1. Stats update - Ignore a datastore during stats update.
2. Ignore a datastore during instance spawn.

Change-Id: Id0cb3f5fdb4d0bc4884cf5405fadc433d4a2b6ba
Closes-Bug: #1330065
Libvirt: Do not raise ENOENT exception

If the disk disappears during update_status(), only need to print
a warning, and do not need to raise an exception.

Change-Id: I87800c855c7b349f69163e33031404ad25e2a9ca
Closes-Bug: 1358609
Allow three periodic tasks to hit slave

Allows the following periodic tasks to hit a slave
if a slave_connection is configured:

_run_pending_deletes
_instance_usage_audit
_poll_volume_usage

Blueprint: juno-slaveification
Change-Id: I38585562e3d3579299e32b5f9ced9311f672abcc
Merge "VMWare Driver - Ignore datastore in maintenance mode"
Merge "Allow three periodic tasks to hit slave"
Merge "Libvirt: Do not raise ENOENT exception"