Driver does not populate IPAddress field in BaseDriver struct

[janeczku]

Elastic IP Address should be populated in the IPAddress field of the Driver struct. See https://github.com/docker/machine/blob/d889abc3528b09753746caaa79bb943bc835c20f/libmachine/drivers/base.go#L17.

[x1022as]

make sense to me.

but Driver struct gets both ElasticIPId and ElasticIP fields in it. these two fields seem related, it will be much more clear to put them together.

Any particular reason to use IPAddress?