vps/models.py: allow VPS init if we have any IP

Currently, if no IPv4 addresses are available for a VPS, the VPS will
not be initialised at all and it will 'fail fast'.  This patch will
allow a VPS to run with only a IPv6 address.
Merge pull request #7 from Centarra/vps-allow-v6-only

vps/models.py: allow VPS init if we have any IP