Added variable $CONFIG_DBTABLEPREFIX in configuration file for table prefix

Table prefix is useful where only one DB is possible (cheap hosting).
this way is possible define something like a "namespace"

default is 'oc_'

config/config.sample.php

@@ -8,4 +8,5 @@ $CONFIG_DBHOST='localhost';
 $CONFIG_DBNAME='owncloud-db-name';
 $CONFIG_DBUSER='user-name';
 $CONFIG_DBPASSWORD='password';
+$CONFIG_DBTABLEPREFIX = 'oc_';
 ?>