Fixing mongodb_uri in NBI/LCM for storage

NBI and LCM require the mongodb_uri to share the files between them.
This wasn't being setup when the charm was provided with a string for
an external MongoDB instance. This commit fix that.

Change-Id: Ie70f7033bd32f9c590a54efc216913098bc3356c
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>