Allow requesting serial signing

Allow requesting serial signing

Sigul-signing directories containing a lot of large artifacts may not be
optimal, because whether serial or parallel, the same amount of
network IO is required and the job takes about the same amount of time
in the end. This allows passing "serial" as a third argument to sign_dir
to specifically request serial operation.

Change-Id: I6a6267090cee6cbed1f134b27eb6cec485fa1793
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>