Don't allow inputs to overwrite host tag

closes #1054

This affects tags in the following plugins:

- cassandra
- disque
- rethinkdb

plugins/inputs/rethinkdb/rethinkdb_server.go

@@ -97,8 +97,8 @@ func (s *Server) getServerStatus() error {
 
 func (s *Server) getDefaultTags() map[string]string {
 	tags := make(map[string]string)
-	tags["host"] = s.Url.Host
-	tags["hostname"] = s.serverStatus.Network.Hostname
+	tags["rethinkdb_host"] = s.Url.Host
+	tags["rethinkdb_hostname"] = s.serverStatus.Network.Hostname
 	return tags
 }
 

plugins/inputs/rethinkdb/rethinkdb_server_test.go

@@ -20,8 +20,8 @@ func TestGetDefaultTags(t *testing.T) {
 		in  string
 		out string
 	}{
-		{"host", server.Url.Host},
-		{"hostname", server.serverStatus.Network.Hostname},
+		{"rethinkdb_host", server.Url.Host},
+		{"rethinkdb_hostname", server.serverStatus.Network.Hostname},
 	}
 	defaultTags := server.getDefaultTags()
 	for _, tt := range tagTests {