Fixes VirtualIP to_dict method to use vip id if available

[vaibhavbansal]

No description provided.

[coveralls]

Coverage increased (+3.85%) when pulling 7057772 on vaibhavbansal:working into e775281 on rackspace:working.

[EdLeafe]

Can you explain what problems you were seeing before this change? And why it's better to return just the id, instead of something like:

    dct = {"type": self.type, "ipVersion": self.ip_version}                         
    if self.id:                                                                     
        dct["id"] = self.id                                                         
    return dct                                                                      

[vaibhavbansal]

Is case we want to share the vip with another lb and we pass in virtual_ips as a parameter the to_dict was ignoring the id. As a result everytime a new vip was getting generated for every loadbalancer. According to the LBaaS api doc if we want to share the vip we need to pass only the vip id. I haven't tried passing all the data along with the vip. But i think it will just get ignored as the id is enough for the LBaaS service to uniquely identify a vip