box: fix unauthorized inserts into _truncate table

Non privileged user (thru public role) has write access to _truncate
table in order to be able to perform truncates on it's tables. Normally
it should be able to modify records only for the tables he has write
access. Yet now due to bootstrap check it is not so.

Closes tarantool/security#5

NO_DOC=bugfix

(cherry picked from commit 941318e7)