Feature Request: Disable SSH

# Current

ParallelCluster always deploys the security of the headnode with port 22 open. The default being open for 0.0.0.0/0. Best practises is typically not using SSH at all and instead use SSM or similar.

# Feature Description

https://docs.aws.amazon.com/parallelcluster/latest/ug/HeadNode-v3.html#HeadNode-v3-Ssh

Add an optional parameter `Enabled` to `Ssh`:

```
Ssh:
KeyName: string
AllowedIps: string
Enabled: boolean
```
The default should be `true` to be compatible with the previous setting. If `Enabled` is given and has the value `false`, then `KeyName` and `AllowedIps` may not be provided. The security of the head node will not get an entry for SSH.

# Current workaround

Use `0.0.0.0/32` as AllowedIps. But it adds to confusion and (potentially) to some security risk.