Backup restore silently overwrites index dir if test instance coexist with production system

Steps to reproduce behaviour:

1. Set up a test instance on the same host as already running JIRA (production) system.
2. Create an XML backup in the production system.
3. Attempt to restore it onto the test system.
   The following page is displayed:
   

Note, there is no enough warning that the restore process is about to reuse the existing index directory and associate it also with the test instance. Running two JIRA instance against index is known to cause all sort of obscure behaviour of the application.

We should implement a check if the directory is already in use and ask the user to confirm his choice or move index to 'jira home', similarly to the invalid path warning: