Get the kubeconfig credentials from MongoDB

This the previous approach, the kubeconfig was gotten from the filesystem,
which has issues scaling the LCM, because the kubeconfig was only present
in the pod that had initialized the environment.

The kubeconfig is stored in Mongo, and this commit basically gets the config from there, so it doesn't matter which LCM needs access to it.

Change-Id: I80458db5124122a4b7b3eb3a9b00cb4a4add11ff
Signed-off-by: David Garcia <david.garcia@canonical.com>