Fix provider problem

With puppet 3.2.2 this gives an error as the default
provider of POSIX doesn't expand the subshell $()

Otherwise you get an error like this: 
Error: Failed to apply catalog: Parameter unless failed on Exec[Server certificate hash]: 'test -f /etc/openldap/cacerts/$(openssl x509 -noout -hash -in /etc/openldap/cacerts/ldapserver.pem).0' is not qualified and no path was specified. Please qualify the command or specify a path. at /etc/puppet/modules/ldap/manifests/server/master.pp:211