Add additional apps directory

[pc-fan]

This is work in progress and tries to address #4.

I added the OWNCLOUD_APPS_DIR var, which defaults to /var/lib/owncloud/apps and gets symlinked to /var/www/owncloud/apps2. In addition owncloud_configure_apps_paths in assets/runtime/functions should change the owncloud config (config based on https://doc.owncloud.org/server/8.2/admin_manual/installation/apps_management_installation.html#using-custom-app-directories). However despite the container starting up correctly and displaying the following log the app_paths var is not added to the config.php. @sameersbn Can you imagine why owncloud_set_param echoes the correct values but they dont appear in the config file?

Container log, fetched with docker logs owncloud-fpm:

Initializing datadir...
Installing configuration templates...
Configuring ownCloud...
Configuring ownCloud::database
Setting up ownCloud for firstrun...
Configuring ownCloud::trusted_domain...
‣ Setting config.php parameter: overwrite.cli.url = 'http://DOMAIN'
Configuring ownCloud::apps_paths...
‣ Setting config.php parameter: apps_paths = 'array ( 0 => array ( 'path' => OC::$SERVERROOT.'/apps', 'url' => '/apps', 'writable' => false, ), 1 => array ( 'path' => OC::$SERVERROOT.'/apps2', 'url' => '/apps2', 'writable' => true, ),)'
Configuring ownCloud::max_upload_size...
Configuring ownCloud::max_file_uploads...
Starting ownCloud php5-fpm...

config/config.php (masked some information with capital letters):

root@4ab3ead6ce1f:/var/www/owncloud# cat config/config.php
<?php
$CONFIG = array (
  'passwordsalt' => 'SALT',
  'secret' => 'SECRET',
  'trusted_domains' =>
  array (
    0 => 'DOMAIN',
  ),
  'datadirectory' => '/var/lib/owncloud/ocdata',
  'overwrite.cli.url' => 'DOMAIN',
  'dbtype' => 'pgsql',
  'version' => '8.2.2.2',
  'dbname' => 'owncloud_db',
  'dbhost' => 'postgresql:5432',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'owncloud',
  'dbpassword' => 'PASSWORD',
  'logtimezone' => 'UTC',
  'installed' => true,
  'instanceid' => 'INSTANCEID',
  'maintenance' => false,
);

Also code to backup installed apps was added, but could not be tested till now. @sameersbn Are the changes in backup_create() and backup_restore() really all it needs to add installed apps to the backup?