[JUJU-736] Fix embedded deploy CLI for external users

[wallyworld]

When using the dashboard CLI widget, the deploy command was failing. The deploy command creates a couple of facades, and calls NewAPIRoot() more than once. Each call does a login and then updates the account details. Because the embedded CLI uses an in memory store for these details, the info was being overwritten and the macaroon was cleared.

This fix does 2 things:

• refactor deploy to only get the api root once
• make the mem store robust to account updates in case it happens again

QA steps

juju bootstrap aws/ap-southeast-2 ian --config identity-url=https://api.jujucharms.com/identity --config identity-public-key=hmHaPgCC1UfuhYHUSX5+aihSAZesqpVdjRv0mgfIwjo=
juju grant wallyworld@external login
juju grant wallyworld@external write default

then use the dashboard to deploy a charm
also check deploy from the CLI

[ycliuhw]

LGTM, just two suggestions ty

[wallyworld]

$$merge$$