MB-11058: lower default concurrent xdcr vb reps to 16

This is temporary until we find out whether 16 really works in
WANs. But right now we need this in order to unblock further testing
of xdcr.

Change-Id: Ib46e9615c93f57f57a9b96d961742ee5a00ef958
Reviewed-on: http://review.couchbase.org/37532
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>