Controller reports no upgrades available from 2.1.2

[Paul Gear]

Two of my long-running controllers have shown the following symptoms when attempting to upgrade:

- Controller running 2.1.2 OK for some weeks/months, using released agent version
- Full connectivity from controller to streams.canonical.com available and manually verified as correctly showing 2.1.3 tools available
- "juju upgrade-juju --dry-run" reports no upgrades available
- "juju upgrade-juju --agent-version=2.1.3" reports no matching upgrades available
- "juju upgrade-juju" upgrades successfully (to 2.1.3.1) by uploading the local copy of the tools

This is very confusing behaviour and could lead users to believe they are fully up-to-date with respect to CVE-2017-9232 when they are not. (I've marked this as a security bug because of this, although I recognise that because it is possible to upgrade to a fixed version, it may be desirable to remove this.)

[Paul Gear (paulgear)]

This may be related to/a duplicate of bugs #1648752 and #1639291.

Session log showing the behaviour is at https://pastebin.canonical.com/189464/

[Paul Gear (paulgear)]

Partial controller debug log is at https://pastebin.canonical.com/189467/

[Dmitrii Shcherbakov (dmitriis)]

My 2 cents:
https://bugs.launchpad.net/juju/+bug/1681853