Groups added with bridge/config/add_group are missing the 'retain' property

[sjorge]

Bug Report

What happened

Added a test group by publishing
test to zigbee2mqtt/bridge/config/add_group, this resulted in

groups:
  '1':
    friendly_name: 'test'

To be added to the configuration (so far so good), however on restart zigbee2mqtt fails to start with the following error:

zigbee2mqtt:error 2019-11-03T19:38:49: Refusing to start, configuration.yaml is not valid, found the following errors:
zigbee2mqtt:error 2019-11-03T19:38:49:   - groups['1'] should have required property 'retain

This makes sense as 'retain' seems to be a required property for groups.

What did you expect to happen

The group to have retain set to either 0 or 1

How to reproduce it (minimal and precise)

Add a new group via mqtt

Debug Info

zigbee2mqtt version: 1.6.0 @ dev from 20191103
CC253X firmware version: n/a

[sjorge]

I'll see if I can fix this while I am adding support for creating a group with an id set.

[sjorge]

I have a fix for this, will open PR for feedback in a bit...

[sjorge]

Now fixed after PR #2266 got merged