MRC: Added support for OSD selection based on custom OSD configuration...

MRC: Added support for OSD selection based on custom OSD configuration parameters. The default OSD filtering policy (ID=1000) now checks if a custom policy attribute was set in order to restricts the selection of OSDs. It only accepts OSDs that have a respective custom configuration parameter.
docs: updated user guide wrt. custom configuration parameters and OSD selection based on these

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@2885 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534