#393)" (#409)"

PR-393 was merged before we can use it properly from pmm-managed.
In order to make it work with basic auth, we need to implement the
changes in pmm-managed/mongodb exporter config to create a new file
having basic auth credentials.

This reverts commit 5e5bf25.

This is intended to be applied on top of #393 (the re-revert) and adds a
new parameter, the tls-config-path file that should be sent by
pmm-managed, pointing to a file containing the credentials we use to
enable basic auth.

In the original implementation, we use exporter_shared library which
receives basic auth credentials via env vars.
Now, using Prometheus Toolkit library, we need to pass a file.