SPARK-1166: clean vpc_id if the group was just now created

Reported in https://spark-project.atlassian.net/browse/SPARK-1166

In some very weird situation (when new created group master_group and slave_group have valid vpc_id), user will receive the following error when running the spark-ec2 script

```

Setting up security groups...
ERROR:boto:400 Bad Request
ERROR:boto:<?xml version="1.0" encoding="UTF-8"?>
<Response><Errors><Error><Code>InvalidParameterValue</Code><Message>Invalid value 'null' for protocol. VPC security group rules must specify protocols explicitly.</Message></Error></Errors><RequestID>fc56f0ba-915a-45b6-8555-05d4dd0f14ee</RequestID></Response>
Traceback (most recent call last):
  File "./spark_ec2.py", line 813, in <module>
    main()
  File "./spark_ec2.py", line 806, in main
    real_main()
  File "./spark_ec2.py", line 689, in real_main
    conn, opts, cluster_name)
  File "./spark_ec2.py", line 244, in launch_cluster
    slave_group.authorize(src_group=master_group)
  File "/Users/nanzhu/code/spark/ec2/third_party/boto-2.4.1.zip/boto-2.4.1/boto/ec2/securitygroup.py", line 184, in authorize
  File "/Users/nanzhu/code/spark/ec2/third_party/boto-2.4.1.zip/boto-2.4.1/boto/ec2/connection.py", line 2181, in authorize_security_group
  File "/Users/nanzhu/code/spark/ec2/third_party/boto-2.4.1.zip/boto-2.4.1/boto/connection.py", line 944, in get_status
boto.exception.EC2ResponseError: EC2ResponseError: 400 Bad Request
<?xml version="1.0" encoding="UTF-8"?>
<Response><Errors><Error><Code>InvalidParameterValue</Code><Message>Invalid value 'null' for protocol. VPC security group rules must specify protocols explicitly.</Message></Error></Errors><RequestID>fc56f0ba-915a-45b6-8555-05d4dd0f14ee</RequestID></Response>

```


The related code in boto is as following:


```
group_name = None
        if not self.vpc_id:
            group_name = self.name
        group_id = None
        if self.vpc_id:
            group_id = self.id
        src_group_name = None
        src_group_owner_id = None
        src_group_group_id = None
        if src_group:
            cidr_ip = None
            src_group_owner_id = src_group.owner_id
            if not self.vpc_id:
                src_group_name = src_group.name
            else:
                if hasattr(src_group, 'group_id'):
                    src_group_group_id = src_group.group_id
                else:
                    src_group_group_id = src_group.id
        status = self.connection.authorize_security_group(group_name,
                                                          src_group_name,
                                                          src_group_owner_id,
                                                          ip_protocol,
                                                          from_port,
                                                          to_port,
                                                          cidr_ip,
                                                          group_id,
                                                          src_group_group_id)

```

So if we just create a new cluster, we should clean the vpc_id for the user