snap known doesn't provide a way to list all assertions

[Ryan Harper]

1. % lsb_release -rd
Description: Ubuntu 16.04.1 LTS
Release: 16.04

2. % apt-cache policy snapd
snapd:
  Installed: 2.16
  Candidate: 2.16
  Version table:
 *** 2.16 100
        100 /var/lib/dpkg/status
     2.15.2ubuntu1 500
        500 http://us.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
     2.0.2 500
        500 http://us.archive.ubuntu.com/ubuntu xenial/main amd64 Packages

3. snap known

known without argument should list all known assertions

4. snap known returns nothing (well help text suggesting it needs an assertion type), additionally it doesn't give any hints as to the types of assertions it might have in its database. If you don't know all possible assertion types, you have no way getting a list of those assertion types.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Julia Palandri (julia-palandri)]

I tried with snap known -h and it doesn't list them either, would be cool to have them handy.