Set home directory for libreoffice conversion

Fixes https://github.com/owncloud/documents/issues/291#issuecomment-48953083

I use owncloud's get_temp_dir() because \OCP\Files::tmpFolder(); would change every call. This would be alot of wasted IO + CPU because the 1MB+ user directory would need to be created repeatedly.
@VicDeo I tried using that af first but it would generate a random folder each time. We want a consistent location so Documents doesn't need to create 1MB+ of files everytime it generates a preview. How about DATADIRECTORY/documents instead? I know the notes add uses DATADIRECTORY/notes for all its stuff.