rm Salt grains.master_ip

This appears in the Salt documentation, is set by Vagrant, but has no
consumers. Remove vestigial references.

cluster/vagrant/provision-master.sh

@@ -74,7 +74,6 @@ EOF
 cat <<EOF >/etc/salt/minion.d/grains.conf
 grains:
   node_ip: '$(echo "$MASTER_IP" | sed -e "s/'/''/g")'
-  master_ip: '$(echo "$MASTER_IP" | sed -e "s/'/''/g")'
   publicAddressOverride: '$(echo "$MASTER_IP" | sed -e "s/'/''/g")'
   network_mode: openvswitch
   networkInterfaceName: eth1