Prevents the etcd/raft-based orderer from writing to the production path
during tests. Instead it uses os.TempDir for WALs and snapshots.

This makes FAB-12824 #done.

Change-Id: I6bcdf5b0ac59575158e6bdb9e8d0e033f926b369
Signed-off-by: Kostas Christidis <kostas@christidis.io>