Support SSH to private IP

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### Tell us about your request

Add an option to `eb ssh` to always SSH to the internal IP if it exists.
OR
If the instance has no public address, SSH to the internal IP (rather than internal hostname).

### Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?

Our instances exist in private networks and the internal hostname cannot be resolved outside. However we have set up a VPN which allows developers to run `ssh 10.1.1.1` from their local machine and connect to EB instances. However, `eb ssh` by default uses the internal AWS hostnames like `ip-10-1-1-1.ap-northeast-1.compute.internal` which do not resolve outside the VPC.

### Are you currently working around this issue?

Wrote our own shell script to do this.

### How are you currently solving this problem?

### Additional context
Anything else we should know?

Place to implement fix:
https://github.com/aws/aws-elastic-beanstalk-cli/blob/ad4f92cf1be6fbb0d2b1e365514a6c5e5c1b144d/ebcli/operations/sshops.py#L86

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