acm: adding 'acm_mode' option to configuration file

acm_mode option determines which mode ACM runs in:
whether the backend is the original ACM mode (aka
ACM version 1) or new SSA mode.

'acm_mode' option can get the following values:
 - acm
 - ssa

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>