core: Added a method to add a CDA message variable

Added a method in CommandBase to add a CDA message variable directly
instead of using various ways of string format and string concatenation
this results in cleaner code and more error prone

Change-Id: Ife678c48bd4ca2422a5ef9c09566c34936edc9df
Signed-off-by: Tal Nisan <tnisan@redhat.com>