PIM-837 : Create a new variant group

Bug fix: [yes]
Feature addition: [yes]
Backwards compatibility break: [no]
Unit test passes: [yes]
Behat scenarios passes: [yes]
Checkstyle issues: [no]*
Fixes the following jira:
 - PIM-836 : Add filtering on axis for variant group datagrid
 - PIM-837 : Create variant group
 - PIM-838 : Edit a variant group (without product datagrid)
 - PIM-839 : Delete a variant group