Add support for communicator SSH

#### Description
The plugin doesn't support the communicator configuration and if I would configure for ansible provisioner ansible with use_proxy set to false I noticed this error:

```
==> lxd.rhel: Warning: use_proxy is false, but instance does not have an IP address to give to Ansible. Falling back to use localhost proxy.
==> lxd.rhel: Warning: use_proxy is false, but communicator is neither ssh nor winrm, so without the proxy ansible will not function. Falling back to localhost proxy.
```

In this case the lxc-exec command fails and the ansible connection remains in hang and I can't connect to the the lxd VM directly via SSH.

#### Use Case(s)

Add the possibility to connect directly via SSH and avoid the usage of lxc exec.

#### Potential configuration

```
source "lxd" "rhel" {
...
communicator = "ssh"
...
}
```

#### Potential References

Start by tracing how the LXD builder handles the communicator configuration and how Ansible provisioning selects lxc-exec versus direct connections. Use the reported use_proxy=false scenario and the proposed communicator = "ssh" configuration as the acceptance case; done means direct SSH connection works without the localhost proxy or lxc-exec fallback.

Written by the indexing model from the issue text.