example SimpleSAMLphp config gives fatal error when using memcache as session_store

[briantully]

Also, we should indicate that using memcache instead of database is probably wise on high-traffic sites

My system information:

• Operating system type: [replace]
• Operating system version: [replace]
• BLT version: [replace]

Output of blt doctor:

When I run this command:

[replace with command]

I get the following output:

[replace with ENTIRE command output. DO NOT enter partial output.]

And I expected this to happen:

[briantully]

PHP Fatal error: Uncaught Exception: /mnt/www/html/mtatodaystg/vendor/simplesamlphp/simplesamlphp/config/config.php: The option 'memcache_store.prefix' is not a valid string value. in /mnt/www/html/mtatodaystg/vendor/simplesamlphp/simplesamlphp/lib/SimpleSAML/Configuration.php:604

This is due to the fact that the $config['memcache_store.prefix'] is set to NULL.

function mc_session_store(array $config) {
  $config['store.type'] = 'memcache';
  $config['memcache_store.servers'] = mc_info();
  $config['memcache_store.prefix'] = $ah_options['database_name'];

  return $config;
}

Note that $config['memcache_store.prefix'] = $ah_options['database_name']; is called however the $ah_options array isn't passed in as an arg to the mc_session_store function.

I'll submit a PR for review.