Do not do mysql_flavor detection for external/unmanaged tablets

[morgo]

Skips the panic for flavor detection for external tablets.

Manually tested with:

/home/morgo/vitess/bin/vttablet -topo_implementation etcd2 -topo_global_server_address localhost:2379 -topo_global_root /vitess/global -tablet-path zone1-0000000102 -tablet_hostname '' -init_keyspace commerce -init_shard 0 -init_tablet_type rdonly  -service_map grpc-queryservice,grpc-tabletmanager,grpc-updatestream -pid_file /tmp/vtdataroot/vt_0000000102/vttablet.pid -vtctld_addr http://morgox1:15000/ -db_host 127.0.0.1 -db_port 5726

Without this patch I will get a panic, which is a regression in behavior introduced by flavor detection:

panic: could not auto-detect MySQL version. You may need to set VT_MYSQL_ROOT so a mysqld binary can be found, or set the environment variable MYSQL_FLAVOR if mysqld is not available locally:
	VT_MYSQL_ROOT: /tmp
	VTROOT: 
	vtenv.VtMysqlRoot(): /tmp
	MYSQL_FLAVOR: 
	

goroutine 1 [running]:
vitess.io/vitess/go/vt/mysqlctl.NewMysqld(0x21a70e0, 0x0)
	/home/morgo/vitess/src/vitess.io/vitess/go/vt/mysqlctl/mysqld.go:165 +0x735
main.main()
	/home/morgo/vitess/src/vitess.io/vitess/go/cmd/vttablet/vttablet.go:120 +0x51a

Signed-off-by: Morgan Tocker [email protected]