After PR #116, it's possible to inject an existing combined_ca_bundle
to the keymanager initialisation. However, it won't work for multiple
users because the file is removed when the keymanager is destroyed.

Considering there's no sensitive data on the file and it's created
as a temp file, we don't need to handle its deletion.

For the multi-tenancy case, the combined_ca_bundle could be done once
to avoid unnecessary disk IO for each user.
The default case is still valid, and will not cause issues anywhere.

- refresh random key in random time
- add get key by fingerprint
- refactor nicknym methods to own file
- tests

- note this do not include a check for
revoked key, since that need some changes
in gnupg

- Related: #6089