Add support for openstack-common::sysctl

There are a number of sysctl settings in the OpenStack documentation that are
not managed by the cookbooks. This approach will iterate over any attributes
added to the node['openstack']['sysctl'] hash and write them out to
/etc/sysctl.d/60-openstack.conf.

The README.md had the recipe description within the Libraries section, separated
into new Recipes section.

Change-Id: Ic32184b78a0bcf4c3e704c7dcd1e9d009b1d95c6
Merge "Add support for openstack-common::sysctl"