No need for a `begin` within a block.

Since there are 2 different branches for #branch_add_keys,
we need to handle 2 different errors but ensure that it
returns the same value (true or false).

This way, the consumer of the method can handle whatever
the case is easily. In this case, it's only being used
in `gitlab:shell:setup`.

This is so that if the authorized_keys_file config is
not yet set (e.g. GDK configuration not updated) things
will still work.

Set `authorized_keys_file` to `nil` as default so we don't
assume its default location.

Will be removed in future releases.

This commit also includes adding documentation for each
public method of the class and changes to the signature
of #batch_add_keys.

It's still being used in gitlab-ee

Set it via gitlab.yml under test environment instead

- Use File#open instead of Kernel#open
- Add spaces inside hashes
- Remove unneeded disabling of Lint/AssignmentInCondition