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 in
the default keyring.
Merge pull request #3662 from grosskur/apt-key-keyring-param

apt_key: Add 'keyring' parameter