add instructions on how to manually install packagecloud

@ahanselka I've added instructions on how to manually install packagecloud deb package. The Azure restore contains everything we need already, so I guess next iteration from here will be just automating all this.

I can work on automating this during next Backup Appreciation Day, and can also test through John's gpg changes. Unless its desirable to do this earlier, what would be your call @northrup ? My point is -- if we have gpg-enabled backups now, can we end up with two weeks of encrypted blobs which are untested? This is my only reasoning behind doing this earlier than July's Backup Appreciation Day (and I wish it had better acronym :)

Closes infrastructure#1917 (closed).