Introduce an environment variable for the configuration of the office server

Problem:

• on our CI infrastructure, we have LibreOffice installed in /opt/libreoffice3.6, which is not a standard path
• JODConverter do not look for the office server at this location
• So our functional tests do not work.
• We cannot change xwiki.properties during the execution of the tests because the tests are supposed to work in any environment, not only our CI.
• This problem can probably be reproduced on other systems.

Solution:

• Introduce an environment variable called XWIKI_OFFICE_HOME.
• Make our office component look at this variable if nothing is set on xwiki.properties.
• This variable is easy to set on our CI (Jenkins have an option for that).
• This variable is easy to set for any user who want to execute the integration tests, without touching their system.