Fix some provisioner and policy prompt issues

[hslatman]

This PR fixes the following issues:

• SCEP provisioners not detected in admin token flows. They now return an error, similar to ACME provisioners.
• Invalid provisioner selection logic when managing provisioner policies. The --provisioner flag was used to select a provisioner to authenticate as well as the provisioner to manage policies for.
• Unexpected error messages showing "issuer" instead of "provisioner" flag. In certain situations the CLI would return error messages indicating an issue with the --issuer flag value, whereas it was actually supplied in the --provisioner flag. This fixes step cli flags with aliases sometimes report a different alias in error messages #821.
• Update some usages of issuer in our code to reflect provisioner names.

[maraino]

It looks good, but I would try to see if doing this would work:

ctx.Set("provisioner", "")
// and perhaps 
// ctx.Set("issuer", "")

[maraino]

It looks a bit hacky, but it would be nice to find a different way to set the flag to "" for just the things we need. Perhaps with token options instead of getting things from the context in this "flow" methods.

[maraino]

Looks good, but I would also set the alias to "". This will also fix the issue if the alias is in an environment variable or on defaults.json

[maraino]

lgtm