Enable to pick IO engines explicitly

Change-Id: I4b7bf3c1b8cfa1ac939da73dcc0039219575acd4
Reviewed-on: http://review.couchbase.org/23654
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>