JIRA/OWD-36588 - Push server: Dynamic configuration of mobile networks for wake up

Discussed offline with Antonio. This is the same insecure method as maintain these keys as unencrypted files.
Since this will be in a controlled environment and exclusive mongodb, we'll create two new issues to improve this in the future, but it's not an urgent task just now.

Discussed with Antonio

Issue [1] is for add credentials to mongo.
Issue [2] is for discussion and possibly add in a near future HSM support with will be a really secure improvement ;)

[1] https://github.com/telefonicaid/notification_server/issues/320
[2] https://github.com/telefonicaid/notification_server/issues/321