MB-100:cbcollect_info: correctly compute installation root

initargs is normally %(root)/var/lib/couchbase/initargs

which is 4 levels above root, not 2 as I previously did

Change-Id: I4ed931de147353de310ed2e045bffcc34c7dce84
Reviewed-on: http://review.couchbase.org/26393
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>