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 matches that which was done in the
galera_client role in I520ccbadf3320b0d07fc83e3dbec9ea2bd16ec83
[1] https://github.com/ansible/ansible/issues/31691
Change-Id: I2715c904975b7940af72bd422904e748d3bae953