EC2: add constraint when querying for IP address

When querying for an IP address after instance creation, only return the
private IP address if it's asked for in the profile. This tends to be a
problem when dealing with instances in VPCs, and it takes a while for
the public IP address to be generated.

Fixes saltstack/salt-cloud#693.
Merge pull request #788 from malept/aws-ec2-vpc-public-ip

EC2: add constraint when querying for IP address