config: Add keyring module with fetch and set.

New config submodule for fetching secrets from a secrets service over dbus. To
facilitate this we add a section and key to the stored attributes, allowing us
to fetch all mopidy values from the keyring and map them to the correct config
values.

A helper for setting values is also added. Due to differences in in the secrets
dbus API across versions of the APIs and various states of support in the
secrets service we try and open the default, login and finally session
keyrings for storage.

Locked keyrings will be dismissed for all operations.
config: Integrate keyring values with overrides.
config: Keyring debug logging cleanup
config: Review fixes.
Merge pull request #449 from adamcik/feature/keyring

Add keyring support via dbus secret service