Fix breakage that occurred adding get for a region

[reedobrien]

Ensure we aren't resetting and getting at the same time -- which is
invalid. When adding the feature to allow getting all settings
for a specific region we removed the check to see if reset was set and
prevent also setting getConfig from being called in Run. This fixes the
issue by putting the appropriate check in place and adding tests to
ensure there is no stdout when resetting.

Fixes: lp:1634197

QA:

• Run the unit tests and ensure they all pass
• juju bootstrap awstest reed/aws/us-west-1 a controller in a cloud with regions, using a config like; awstest here
• juju model-defaults

    Attribute                   Default           Controller
    ...
    logging-config              ""                <root>=TRACE
    no-proxy                    ""                https://local
      us-east-1                 https://foo-east  -
      us-west-1                 https://foo-west  -
    provisioner-harvest-mode    destroyed         -
    ...

• juju model-defaults us-east-1

Attribute    Default           Controller
no-proxy     ""                https://local
  us-east-1  https://foo-east  -

• juju model-defaults --reset ftp-proxy us-east-1 # no output
• juju model-defaults --reset ftp-proxy # no output
• juju model-defaults --reset no-proxy us-east-1 # no output
• juju model-defaults us-east-1
ERROR there are no default model values in region "us-east-1"
• juju model-defaults us-east-1 no-proxy
ERROR there are no default model values for "no-proxy" in region "us-east-1"

[reedobrien]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju