Added password obfuscation for external storage config

Added obfuscation for all "password" options from external storages.
Added more ext storage unit tests config.
Added unit tests for reading/writing the configuration.
Added IV for ext storage password encryption
Moved the mounting code for external storage from
OC\Filesystem::initMountPoint to files_external using the post_initMountPoints hook
Fixed ext storage unit test for groups

Squashed backport of 2c561c9c5072ce82e06bd5ab2c4ee81bc5d09d59 from
master.
Merge pull request #8672 from owncloud/stable6-obfuscatepasswords

[backport] [stable6] Added password obfuscation for external storage config