Support for VPC/subnet selection

Example usage via bosh-bootstrap

```
1. cf (10.10.0.0/16)
2. EC2 Classic (172.31.0.0/16)
3. EC2 only
Choose a VPC: 1

1. cf-bastian (10.10.0.0/28)
2. cf-microbosh-2a (10.10.0.32/28)
...
Choose a subnet: 2

Choose IP |10.10.0.36| 

Confirming: Using address 10.10.0.36
Reusing security group ssh for the VPC
 -> no additional ports opened
Created security group dns_server for the VPC
 -> opened dns_server ports UDP 53..53 from IP range 0.0.0.0/0
Reusing security group bosh for the VPC
 -> no additional ports opened
```

It avoids the unusable and already in-use IPs within a selected VPC/subnet. Security groups can be created into a VPC.