For aws-ssh connect implement modifying security group to add the user's IP address

[ekini]

This will allow EC2 instances to be locked down.

Implementation:

1. if the ec2 instance has aws-ssh-security-group tag, aws-ssh will modify it.
2. alternatively, check for --security-group flag?
3. Add the user's IP address to the security group.

It is expected that the ec2 instance operator will clean up the security group afterwards.