cmd/juju/user: correct padding for user registration string

[4a6f656c]

The current code incorrectly pads the ASN.1 encoded user registration
data with remainder bytes, rather than padding it to a multiple of
three bytes. This results in equal signs being appended for base64
padding.

Also add test coverage that verifies the registration string only
consists of alphanumeric characters.

[babbageclunk]

This looks great, thanks! Could you change the PR to be against develop? The merge bot won't merge things into staging directly. (I'm not sure why that's the default when we create PRs.)

[babbageclunk]

Actually, I got @howbazaar to do it.

$$merge$$

[babbageclunk]

Let's try now.

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Generating tarball failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/10363

[babbageclunk]

gofmt is unhappy with this - line 118 of add_test.go should be:

s.mockAPI.secretKey = []byte(strings.Repeat("X", 32+i))

(No spaces in 32+i.)

I'd fix it but I don't have the privs required to edit the PR.

[4a6f656c]

@babbageclunk Ugh, fixed - thanks!

[babbageclunk]

Sorry, missed that this was updated.

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju