Bluetooth: Mesh: Allow changing cfg client message timeout

10 seconds is quite long for configuration messages, and way too much
currently since we only talk through the local networking interface.
Set the default timeout to 2 seconds, and provide APIs through which
the timeout may be changed at run-time (mainly useful for the shell).

Note: The timeout_set() API is normally assumed to be called just once
for an application, based on the expected size of the network (hops &
latency). Trying to change it e.g. in a multi-threaded environment for
every message may not yield the expected results.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>