SSHDriver to use tools to find ssh/scp/sshfs/scp

[mattyclarkson]

Other drivers use self.target.env.config.get_tool(...) to retrieve a tool path from the configuration.

The SSHDriver uses raw "ssh", "scp", "rsync" and "sshfs" calls.

It would be useful to respect those keys in the configuration tools map. Then OpenSSH binaries can be provided in the configuration that may have been hermetically built by an external system. This would require to add a lookup for those keys in the SSHDriver and update the subcommands to use the result.

Some tools have the following pattern:

        # FIXME make sure we always have an environment or config
        if self.target.env:
            self.tool = self.target.env.config.get_tool('dfu-util')
        else:
            self.tool = 'dfu-util'

The tests for SSHDriver do not always provide a config so would need the same pattern. Should we change the tests to always provide a configuration? Are there legitimate situations that the SSHDriver is used without a configuration?

The keys would be added to TOOL KEYS.