Changes behaviour of diskspace.py how monitored filesystems are collected

Hi,
we had a little trouble with the diskspace collector - it made a stat() call on every file system listed in /proc/mounts, which caused our auto mounter to mount all of them.
To prevent this I moved the filesystem filtering and handling of the exclude list above the stat call. Think this could be useful for others that have autofs running on their hosts.
Additionally I added a line to the mocked proc/mounts-file to test this.

Cheers
Robert