Bug 1991 fixed: modified LCM charm secret name to a standardized one

Bug 1991 fixed: modified LCM charm secret name to a standardized one

Previously, the LCM charm was creating a Kubernetes secret with the name
`lcm-lcm-secret`, to save the credentials for accessing Mongo, while the
other modules' charms where using a name with the format
`<mod_name>-mongodb-secret` to do so. This modification updates the LCM charm
to create this secret in a compliant way with this name format.

Change-Id: I4d91e760e44d1703f246e605d9b70625967556c5
Signed-off-by: Pedro Escaleira <escaleira@av.it.pt>