This adds a new `deployment` configuration attribute (`targetUser`).
To inherit the username from the local user issuing the deployment
set:
```
deployment.targetUser = null;
```
Setting this to a string will deploy as that user. This option
defaults to "root".

 ### We assume the following for non-root deploys:

- Passwordless sudo (no TTY allocation possible)

I'm using the following NixOS configuration
```
security.pam.services.sudo.sshAgentAuth = true;
security.pam.enableSSHAgentAuth = true;
```

- The deployment user is trusted by the Nix daemon
```
nix.trustedUsers = [ "adisbladis" ];
```
This is required because of nix-copy-closure.

Closes #730

…e escalation command with --

This is so that we won't get inconsistencies between different
subcommands like `nixops send-keys` (which doesn't eval) the
deployment attributes.

This change should be reverted at a later date when we have made these
commands evaluate the configuration.

…chine

Also extend `nixops mount` with full support for all SSH arguments.

Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>

Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>

Add support for non-root deployments