Retry lxc-net service start on CentOS7
Retry lxc-net service start on CentOS7

This intermittently fails to start, we should retry the service start,
to reduce the impact.

Change-Id: I15e5908ba5b576bb1a0bceba30e42fb80b91e90c

openstack/openstack-ansible-lxc_hosts,openstack/openstack-ansible-lxc_hosts