[juju] Adds 'juju get' output, exposes export-mongodb

This commit extends the plugin to include the resulting 'juju get
${service}' for each deployed service, also it exposes the
export-mongodb option, which includes on the report a json file
containing each data collection used by juju-core.

Signed-off-by: Jorge Niedbalski R <niedbalski@gmail.com>
Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>