Allow host user creation when host_groups contain login name

[eriktate]

Fixes #35142

This adds some additonal logic around resolving a primary GID while creating new host users. The expected invariants are:

• A host user should have a primary group generated for them if it doesn't already exist and their username isn't present in host_groups.
• A host user should be assigned the existing GID of the group sharing their username if it's present in host_groups.
• A host user should fail creation if their generated primary group already exists, but isn't present in host_groups.

I'm not entirely sure that last one should be included, but it seemed safer not to automatically assign a user to a group that isn't also managed/mentioned in the role. This can be circumvented by either adding the group name to the host_groups in the role, or by explicitly assigning a GID to the user's traits.

changelog: Fixed an issue that prevented host user creation when the username was also listed in host_groups.

[public-teleport-github-review-bot]

@eriktate See the table below for backport results.

Branch	Result
branch/v14	Failed
branch/v15	Failed
branch/v16	Create PR