Custom repository creation fails with error "500 - ValidationError (Distributor:None) (Invalid dictionary key name - keys may not contain "." or "$" characters: ['config'])"

Issue

• When trying to create custom repository on RHUA server it fails with following error:

[...]
The following repository will be created:
  ID:              custom-client-rhui-7-x86_64
  Name:            custom-client-rhui-7-x86_64
  Path:            custom/client/rhui/x86_64/7
  Entitlement:     custom/client/rhui/$basearch/7
  GPG Check        Yes
  Custom GPG Keys: '/root/rpm-gpg/rhui-client-rhui.gpg', '/root/rpm-gpg/rhui-client-all.gpg'
  Red Hat GPG Key: Yes
Proceed? (y/n) y


An unexpected error has occurred during the last operation.
More information can be found in /root/.rhui/rhui.log.

Entries from /root/.rhui/rhui.log file on RHUA server:

[...]
+ /usr/lib/rpm/redhat/brp-strip-static-archive /usr/bin/strip
+ /usr/lib/rpm/brp-python-bytecompile /usr/bin/python 1
+ /usr/lib/rpm/redhat/brp-python-hardlink
+ /usr/lib/rpm/redhat/brp-java-repack-jars
+ umask 022
+ cd /root/clientrpm/rhui-client-2.0/build/BUILD
+ cd rhui-client-2.0
+ rm -rf /root/clientrpm/rhui-client-2.0/build/BUILDROOT
+ exit 0

2016-11-29 15:46:25,440 - <class 'pulp.bindings.exceptions.PulpServerException'>
2016-11-29 15:46:25,440 - Unexpected error caught at the shell level
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/rhui/tools/shell.py", line 88, in safe_listen
    self.listen(clear=first_run)
  File "/usr/lib/python2.7/site-packages/rhui/tools/shell.py", line 122, in listen
    Shell.listen(self)
  File "/usr/lib/python2.7/site-packages/rhui/common/shell.py", line 186, in listen
    item.func(*args, **item.kwargs)
  File "/usr/lib/python2.7/site-packages/rhui/tools/screens/repo.py", line 525, in create_custom
    rh_gpg_key)
  File "/usr/lib/python2.7/site-packages/rhui/tools/pulp_api.py", line 783, in create_custom_repo
    distributors)
  File "/usr/lib/python2.7/site-packages/pulp/bindings/repository.py", line 81, in create_and_configure
    return self.server.POST(path, repo_data)
  File "/usr/lib/python2.7/site-packages/pulp/bindings/server.py", line 100, in POST
    log_request_body=log_request_body, ignore_prefix=ignore_prefix)
  File "/usr/lib/python2.7/site-packages/pulp/bindings/server.py", line 166, in _request
    self._handle_exceptions(response_code, response_body)
  File "/usr/lib/python2.7/site-packages/pulp/bindings/server.py", line 204, in _handle_exceptions
    raise exceptions.PulpServerException(response_body)
PulpServerException: RequestException: POST request on /pulp/api/v2/repositories/ failed with 500 - ValidationError (Distributor:None) (Invalid dictionary key name - keys may not contain "." or "$" characters: ['config'])