Store k8s controller information in Mongo, and remove controller attribute from K8sJujuConnector

- Storing the k8s controller information in Mongo let us connect to that controller from different LCM units
- The cacert and secret keys are encrypted
- The controller attribute in K8sJujuConnector was preventing to have more than 1 connection to different controllers at the same time

Change-Id: Icf96e1eedf84e283dd6b0016cae3695a074b7d24
Signed-off-by: David Garcia <david.garcia@canonical.com>