MB-9699: collect raw /proc/uptime

Change-Id: I54bf43fbd0a4f12e3564897d9e0dbf2104a85b2d
Reviewed-on: http://review.couchbase.org/33151
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>