Correct properties of the cluster's file storage dir

Correct properties of the cluster's file storage dir

This patch does:
- Rename the haskell opcode parameter
  'pGlobalFileStorageDir' and 'pGlobalSharedfileStorageDir'
  to 'pClusterFileStorageDir' and
  'pClusterSharedfileStorageDir', respectively, because
  the old name was misleading.
- The two parameters are made optional strings instead of
  non-empty strings. This actually used to be like this
  before, but was accidentally overriden by the switch to
  opcode generation from haskell to python.
- The ClusterFileStorageDir parameter had to be renamed in
  Haskell to not clash with the FileStorageDir parameter
  of the OpInstanceCreate code.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>