Add virtual_ip_assigned? to service object

This can be used to know if a VIP is already assigned; this information
can then be used to avoid triggering a DNS update for the VIP.

(cherry picked from commit 387e26cbaadfc30daab6015063c2110deddc963a)
Merge pull request #277 from vuntz/vip_assigned

Add virtual_ip_assigned? to service object