Fix security group race condition while listing and deleting rules

Previously, there was a race condition that could occur if one was listing
a security group which contained a rule which referenced another security
group as an additional api call is used to look up the security group's
name which is returned (rather than id) via the API. The problem occurs if
this security group is deleted in which case a 404 was raised.
This patch fixes this issue by catching the 404 and ignoring the rule as
its already deleted as there is a constraint enforced that you cannot
delete a security group if another security group has a rule that references
that security group.

Change-Id: I3855b8d3a1bd2f7c88901da071f9bd5581bd56e4
Closes-bug: 1262566
VCDriver - Ignore host in Maintenance mode in stats update

The VCDriver during stats update checks only for connected hosts.
However hosts which are in connected state to the Cluster can
be set to Maintenance mode.
The cluster does not boot instances to hosts which are in
maintenance mode.
Hence the VCDriver should check for hosts which are in
maintenance mode and ignore them for stats update.
VISDK API returns True for the property inMaintenanceMode for
Hosts which are in maintenance mode

Change-Id: Ieaf7530ed2d98c59c5450e8cb75e209823193d49
Closes-Bug: #1316674
Catch InstanceInfoCacheNotFound during build_instances

instance.refresh() is raising InstanceInfoCacheNotFound which is logged
by the conductor and would cause all following builds to fail in a
multiple build request situation.  The cause appears to be a race
condition which should be addressed separately, but for now this
exception should be caught, logged, then ignored.

Change-Id: Ibf9e6687933ee90dad54b1998b243ef7256d79ca
Closes-bug: 1324793
Merge "Catch InstanceInfoCacheNotFound during build_instances"
Merge "Fix security group race condition while listing and deleting rules"
Merge "VCDriver - Ignore host in Maintenance mode in stats update"