DB clients should allow only https when Abacus is secured

[hsiliev]

Currently bosh couch and mongo DB clients allow http and https and self-signed certificates even if Abacus is secured.

We should only use https and disable self-signed certificates, hardening Abacus by default. This makes misconfiguration harder and reduces the attack surface.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/129479897

The labels on this github issue will be updated when the story is started.

[hsiliev]

Self-signed certs using mongoclient are now disabled with b6a1a3c