Let user handle mounts (Azure chroot)

_This issue was originally opened by @backerman as hashicorp/packer#9657. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

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#### Description

The Azure chroot builder should allow the user to handle all mounting of filesystems with `pre_mount_commands`.

#### Use Case(s)

I'm building FreeBSD images with ZFS root in Azure (Ubuntu or another Linux distribution should have the same issue and solution). The Azure chroot builder has no option to *not* mount the root filesystem, which isn't going to work because ZFS doesn't use the `mount` command that way.

#### Potential configuration

```json
{
"variables": {
},
"builders": [
{
"type": "azure-chroot",
"from_scratch": true,
"image_resource_id": "/subscriptions/{{vm `subscription_id`}}/resourceGroups/{{vm `resource_group`}}/providers/Microsoft.Compute/images/freebsd-{{timestamp}}",
"os_disk_size_gb": 8,
"os_disk_storage_account_type": "Premium_LRS",
"os_type": "Linux",
"mount_root": false,
"mount_path": "/tmp/packer-chroot",
"pre_mount_commands": [
"mkdir /usr/freebsd-dist",
"env BSDINSTALL_DISTSITE=https://download.freebsd.org/ftp/releases/amd64/12.1-RELEASE/ DISTRIBUTIONS=\"base.txz doc.txz kernel.txz src.txz\" bsdinstall distfetch",
"bsdinstall script bsdinstall",
"zpool import -R /tmp/packer-chroot zroot"
],
"post_mount_commands": [
"zpool export zroot"
]
}
],
"provisioners": [
{
"inline": [
"env ASSUME_ALWAYS_YES=YES pkg bootstrap",
"/usr/sbin/waagent -force -deprovision+user && export HISTSIZE=0 && sync"
],
"inline_shebang": "/bin/sh -x",
"type": "shell"
}
]
}
```

#### Potential References