We make remote network hits to get the GPG keys which are quite
unreliable, and apt_key does not support using a proxy properly [1]
so let's store them inside the role and use them.
The implementation here is derived from that which was done in the
galera_server role in I9443f10e8c803599cbebfc2a53cb9c432bfa60d1,
but opts to use a mechanism that will be simpler to maintain.
[1] https://github.com/ansible/ansible/issues/31691
Change-Id: I520ccbadf3320b0d07fc83e3dbec9ea2bd16ec83
6c7aee86f5