Cache folder is now configurable

When using an external cache folder, it is automatically mounted in
FileSystem::initFileSystem so that any app can use it transparently
by creating a view on the "/$user/cache" directory.
Added unit tests for external cache folder
Remove unused setUserVars utility method

That method was moved to the external storage recently so
isn't needed here any more.
Merge pull request #7829 from owncloud/cachefolderlocation

Cache folder is now configurable