Refresh network info cache for secgroups

Before updating security group rules, we need to make sure that
the info cache is up-to-date. Without this source groups are not
updated properly. This was a regression introduced in commit
85aac04704350566d6b06aa7a3b99649946c672c which fixed a potential
DOS using source groups.

Fixes bug 1216720

Conflicts:
	nova/network/manager.py
	nova/tests/network/test_manager.py

Change-Id: I6b5115df53f2e159ea506ef966cd49cedd35f83d
(cherry picked from commit 8679b2c8e7f9fddc31a74ad00f6705bca00a762b)
fallocate image only when user has write access

Libvirt will change instance's ownership when the instance became power
off which will make nova-compute service unable to hard reboot instance
bacause the nova-compute service has not write access. This commit will
allocate space for image only the first time we create image.

fixes bug 1200113

Change-Id: Id81b09ade5f870665f5ffabab3cde144c6dec210