selftests: Add new option for client-side passwords

Add new option to nettest to specify MD5 password to use for client
side. Update fcnal-test script. This is needed for a single instance
running both server and client modes to test password mismatches.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>