Honor ssh_private_ip flag in EC2-Classic, not just VPC

VpcId will be nil in Classic, but we may still wish to ssh to the
instance's private IP address -- if for example we are using security
groups to block SSH access via the public IP.

builder/amazon/common/ssh.go

@@ -23,6 +23,8 @@ func SSHHost(e *ec2.EC2, private bool) func(multistep.StateBag) (string, error)
 				} else {
 					host = *i.PrivateIpAddress
 				}
+			} else if private {
+				host = *i.PrivateIpAddress
 			} else if i.PublicDnsName != nil && *i.PublicDnsName != "" {
 				host = *i.PublicDnsName
 			}