Add group permission for /etc/nova directory

Having a service program need to access /etc/nova/nova.conf file,
although the service program's service user has been added into the
"nova" service user, because /etc/nova folder permission has been set to
700 in nova-common recipe, thus the service program can not start. From
openstack-block-storage cookbook, it sets the /etc/cinder directly
permission as 750, I would think it's reasonable to grant the group
access to /etc/nova directory.

Closes-Bug: #1293956
Change-Id: I6d3ee5c424756fe11cefd425a3dddd3595b025d8