Update nova::config example description.

The original example in nova::config is syntax invalid.
Besides fixing that, this patch also gives an example in yaml format.

Change-Id: I2f044392becd9e46cc8346030095a029464c5ea0
Closes-bug: #1301700
Merge "Update nova::config example description."
Merge git://github.com/puppetlabs/puppetlabs-nova