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.
you mean save `get_temp_dir` to a variable then use escapeshellarg?
I can do this tonight when I get back from work;otherwise feel free to make the changes if you'd like. We also need this line added to the conversion test on the admin page.