Parameterize libvirt_cpu_mode

The libvirt_cpu_mode setting in nova.conf currently isn't exposed
via Puppet, but can be very useful in nested virtualization
situations (as are common in development labs).  This patch adds
a parameter and defaults it to 'false'.  If a non-false parameter
is provided, the setting will be added to the [DEFAULT] section
of the nova.conf file.

Implements: blueprint expose-hw-virtualization