Extend Text format by marking the master node

Sometimes, e.g., for hroller, it is necessary to know which node
is the master node. Therefore this information has to be included
in the text format as well. Since we never use an offline node as
master node, we can put this information can be put in the "is
offline" field; as old versions only check this field for equality
to "Y", using "M" to indicate the master is compatible in both
directions. This also leaves room to add more roles in the future,
should we need them.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>