server: Allow to set "none" as default replication policy with a replication factor of 1.

Before this commit the following call falled with the message:

$ xtfsutil ~/mnt/ --set-drp --replication-policy none
The XtreemFS call failed: The server hostname.zib.de:32636 (2dca8a77-5c56-4442-b6eb-15dc35446733) denied the requested operation. Error Value: POSIX_ERROR_EPERM Error message: a default replication policy requires a replication factor >= 2 (errno=POSIX_ERROR_EPERM)
FAILED


git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3248 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534