apt_key: Add 'keyring' parameter

The `apt-key` command takes an optional `--keyring` parameter representing the path to a specific GPG keyring to operate on. If it's not given, the command operates on all keyring files, *i.e.*, `/etc/apt/trusted.gpg` and `/etc/apt/trusted.gpg.d/*.gpg`.

This change adds a `keyring` parameter to the `apt_key` module and propagates it down to the `apt-key` command line. The main use case this supports is organizing keys for third-party repos into individual keyrings in `/etc/apt/trusted.gpg.d`, rather than putting them all inthe default keyring.