Merge lp://staging/~jorge/charms/trusty/juju-gui/add-default-values into lp://staging/charms/trusty/juju-gui

Thanks for the changes Jorge.

Unfortunately this version does not pass charm-proof, which is a fault of the proofing tool. Running it I see:

[juju-gui]bac@trusty64:~/charms/trusty/juju-gui$ charm-proof .
I: config.yaml: option password has no default value
I: config.yaml: option ssl-key-contents has no default value
I: config.yaml: option ssl-cert-contents has no default value
I: config.yaml: option login-help has no default value

Null should be a valid default value for strings. It is treated as None when parsed in Python. The tool is satisfied if you use:

default: ""

But the empty string is not the same as None. In our charm we have tests like:

if login_help is None: # Use the default value

Setting the default to the empty string causes this test to fail and no login help is shown.

If we insist that a 'default: ' key be provide for each configuration value then the tool should be updated to accept a null value as represented by:

default:
default: null
default: ~

See http://yaml.org/type/null.html for more details.

If the tool is already being updated or you have more information please let me know.