At boot time, a CBRAIN app should touch() some filesystem config dirs and files

[prioux]

Some Bourreaux systems are configured with disk allocations where files older than N days are erased automatically.

To prevent such system from deleting the top-level directories for the DP_Cache, and some cbrain-specific admin files, I suggest that part of the boot process should touch them to reset their timestamps.

On a portal or bourreau:

• the DataProvider cache dir
• the DP_Cache_Key.md5 and DP_Cache_Rev.id located in that cache dir

On a bourreau:

• the gridshare dir
• the DP_Cache symbolic link located in it. WARNING: make sure to update the timestamp of the symlink itself!