Fixes lp#1848104: updating existing credential with invalid value invalidates other credentials

[anastasiamac]

Description of change

On the controller facades, creating cloud call context in the facade construction makes no sense since call context is model specific.

This PR changes the creation of the context to be in the actual facade call methods. All context creation has been moved to the calls that use context rather than forcing it to happen too early and for every call.

QA steps

steps in bug report do not result in the controller model cloud credential to be invalidated.

Bug reference

https://bugs.launchpad.net/juju/+bug/1848104

[anastasiamac]

api call context need to be relevant to model

[wallyworld]

A couple of questions about how state is gotten

[wallyworld]

Looks good to go.
Main comment is to use st directly instead of m.State()
(we used st to get Model() in the first place)
Or m.UUID() in one place.

[anastasiamac]

$$merge$$

[anastasiamac]

fatal: The remote end hung up unexpectedly
17:13:38 fatal: early EOF
17:13:38 fatal: index-pack failed
17:13:38 : command failed: [git clone --recursive -v --progress https://code.googlesource.com/google-api-go-client /home/ubuntu/go/pkg/dep/sources/https---code.googlesource.com-google--api--go--client]: exit status 128
17:13:38 Makefile:74: recipe for target 'dep' failed
17:13:38 make: *** [dep] Error 1

[anastasiamac]

unrelated failure in github.com/juju/juju/worker/raft/raftflag.TestPackage

[anastasiamac]

$$merge$$