Allow one group in a group configuration (BLD-1223)

Allow one group in a group configuration (instead of requiring two)

Issue: [BLD-1223](https://openedx.atlassian.net/browse/BLD-1223)

**Acceptance criteria:**
* remove the restriction that a group configuration must have 2 groups.
* still validate to make sure at least one group is present

**Sandbox:** http://studio.olmar.m.sandbox.edx.org/

**Reviewers:** @tymofij @cahrens @explorerleslie