Add new cluster parameter primary_ip_version

We expose the ip_version (4, 6) to the external interface and internally
we convert it to ip_family (AF_INET=2, AF_INET6=10). This makes the code
more concise as all functions deal with family rather than version.

This patch does not yet expose this parameter via gnt-cluster, but only uses
the constant IP4_VERSION. This will be enabled in a future patch.

Signed-off-by: Manuel Franceschini <livewire@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>