lxc-busybox: make some OpenSSH tools optional

lxc-busybox: make some OpenSSH tools optional

Currently, when installing OpenSSH in a Busybox container, the template searches
for all the OpenSSH client binaries available in the Debian distro package. The
included tools might differ from distro to distro, so make part of the tools
optional. The mandatory tools, without which installing OpenSSH fails, are
"sshd" for the server and "ssh" and "scp" for the client.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>