Change CHAP secret default length

Some of the iSCSI initiators have a limit regarding the maximum
CHAP secret length. For example, the MS iSCSI Initiator
does not allow CHAP secrets longer than 16 characters, smaller
than the actual default 20 characters length.

This patch simply changes the default length to 16 characters,
value which is already used by default by some of the volume
drivers. In fact, the iSCSI specs state that: "Implementations
MUST support use of up to 128 bit random CHAP secrets".

Change-Id: I0295fabd0c0048c93e1f452077d0f5d19af9784d
Closes-Bug: #1392792
(cherry picked from commit 3e124edb5dbbd5b48bb8816e6c1b60f5a0fa03db)