RPC 'GetMasterInfo' returns several fields, namely, 'master_netdev',
'master_ip', 'master_netmask', 'master_node', and 'primary_ip_family',
of which only the 'master_node' is actually used.

In this patch:
* remove all the other fields and keep only the 'master_node' field.
* rename RPCs, helper functions (e.g., 'master_info' to 'master_node_name')
* simplify the the voting algorithm code

Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>