introduced pre db open hook

So that when entire thing is run we can catch and refuse attempts to
open vbuckets as couchdb databases.

Change-Id: I08547d29d94d425263370cb3d84dacd8c1270c49
Reviewed-on: http://review.couchbase.org/39473
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>