Require "primary" key in sign_orgs config

[mbonish]

Description
Using "primary" as the primary connection ID lets us define target groups in the group mapping without the "org::" prefix. But right now the config handler does not require "primary" be present in the "sign_orgs" keys. So if a connection is defined like the following, the group mapping may not work as expected.

Steps to reproduce

• Relevant config options: sign-sync-config.yml
• Command-line options used:sign_sync

• Specific details around identity source, group mapping, etc.

Expected behavior
The config handler should raise an error if sign_orgs.primary is not present in the Sign sync config.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment

• UST version: 2.7.0rc3
• OS type and version: windows