Update config.yaml documentation for 'default:'

Current config.yaml documentation implies that default is optional, yet
omitting this field will cause charm proof to complain. Update doc to
describe the default option better.
Merge pull request #165 from kwmonroe/defaultUpdate

Update config.yaml documentation for 'default:'
First draft of all juju environment variables

To address lp bug #1227442
Reorg by api server and unit
add Juju environment variables to navigation
Merge pull request #167 from AdamIsrael/juju-environment-variables

Juju environment variables