Added ext storage unit tests for writing then reload the mount config
Added password obfuscation for external storage config

Added obfuscation for all "password" options from external storages.
Added unit tests for reading/writing the configuration.
Added IV for ext storage password encryption
Moved external storage mounting code to files_external app

Moved the mounting code for external storage from
OC\Filesystem::initMountPoint to files_external using the
post_initMountPoints hook