lxc-busybox: add OpenSSH support

lxc-busybox: add OpenSSH support

Add an additional template parameter for SSH support in the container. Currently
this can be implemented using the Dropbear or OpenSSH utility. The respective
tool needs to be available on the host Linux.

If the parameter is omitted, the template will look for the Dropbear utility on
the host and install it if it is available (legacy behavior).

Adding OpenSSH support has been done following the model in the lxc-sshd
template.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>