Add diag as verification of patches as post-installation/upgrade sanity checks

It would be helpful to introduce a quick post upgrade sanity check in the form of running `riak-admin diag` on each node after upgrading.

It'd also be good to include a bit about checking the `basho-patches` directory for any patches which might not be desired for the new version and cleaning those out as recommended in #406.

The following docs would be affected:

* [Post Installation Notes](http://docs.basho.com/riak/latest/tutorials/installation/Post-Installation/)
* [Rolling Upgrades](http://docs.basho.com/riak/latest/cookbooks/Rolling-Upgrades/)
* [Rolling Upgrade to Enterprise](http://docs.basho.com/riakee/latest/cookbooks/Rolling-Upgrade-to-Enterprise/)