add default relation keys in relation databags

[PietroPasotti]

At the moment, all databags are born empty. They should have the default keys present and set to some sensible defaults. e.g.:

egress-subnets   10.152.183.30/32              
ingress-address  10.152.183.30                 
private-address  10.152.183.30                 

[PietroPasotti]

@benhoyt @sed-i @Abuelodelanada I'm thinking of implementing it in an opt-in fashion:

• add a scenario.STRICT_DATABAGS = os.getenv('STRICT_DATABAGS', False) global
• add a warning that in 3.3 this will default to True and your tests may break
• if STRICT_DATABAGS:
  • all 'null' databags created by e.g. scenario.Relation(remote_unit_ids=[0,3,4]) will contain the default juju keys
  • add a post-init validation on scenario.RelationBase verifying that all databags contain the default juju keys.
  • add a inject_juju_relation_data helper to quickly add defaulted juju keys to a dict.

What do you think?