Please review: [Bug 744945] nsslapd-counters attribute value cannot be set to "off"

[Noriko Hosoi]

https://bugzilla.redhat.com/show_bug.cgi?id=744945

https://bugzilla.redhat.com/attachment.cgi?id=527345&action=diff
https://bugzilla.redhat.com/attachment.cgi?id=527345&action=edit

Description: nsslapd-counters (cn=config) is allowed to have the
value "off".  Once it's set, cache monitor would not be available.
. when staring the server, following message is logged.
   cache_init: slapi counter is not available.
. ldapsearch cache statistics under cn=monitor,cn=<BACKEND>,cn=ldbm
   database, cn=plugins,cn=config shows 0's:
     entrycachehits: 0
     entrycachetries: 0
     entrycachehitratio: 0