Remove unused libvirt_ovs_bridge flag

Progresses blueprint libvirt-opts-group.

Change-Id: I72852fdfdf1bf5e56d3d29c3fecfc87a7b95794a
DocImpact: the unused flag libvirt_ovs_bridge has been removed.
Remove reduntant call to update_instance_info_cache

get_instance_nw_info calls _get_instance_nw_info,
which is decorated with @refresh_cache. This in
turn calls update_instance_info_cache, again. This
is both expensive and more importantly dangerous because
the method update_instance_info_cache may call
_get_instance_nw_info itself, thus causing an infinite
loop.

Change-Id: Id8f0241d1c6609cf4e064ee6855228ae7de250e2
Partial-bug: 1235435
Merge "Remove unused libvirt_ovs_bridge flag"
Merge "Remove reduntant call to update_instance_info_cache"