logging: avoid overriding default encoder
logging: avoid overriding default encoder

With this kind of configuration:

  backends:
    - syslog
    - stderr
  syslog:
    encoder: json

The stderr backend will also have a json encoder.

This patch fixes this.

lebauce/skydive,lebauce/skydive,skydive-project/skydive,safchain/skydive,skydive-project/skydive,skydive-project/skydive,safchain/skydive,skydive-project/skydive,safchain/skydive,skydive-project/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive,safchain/skydive,lebauce/skydive,safchain/skydive,safchain/skydive,skydive-project/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive