Add support for DH groups 14-26

Here's a patch to add support for additional Diffie-Hellman groups.

I've tested to ensure that IKE SAs establish successfully with each of the new groups between two VyOS 1.0.3 instances. The only anomaly is that strongSwan fails to parse the generated ipsec.conf if the `pfsgroup` parameter is set to the [keyword](http://wiki.strongswan.org/projects/strongswan/wiki/IKEv1CipherSuites) for groups 22-24, so I've omitted them from the `esp-group pfs` node template.