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.
oops, I should read your explanation before looking into the code.
This openoffice dir seems to have nothing private inside so `get_temp_dir` is good too.
escapeshellarg is needed here though