PIM-1957: Move Channel constraint to BaseConnectorBundle

Bug fix: [no]
Feature addition: [no]
Backwards compatibility break: [yes]
Unit test passes: [yes]
Specs passes: [yes|no]
Replaced unit test classes removed: [no]
Behat scenario passes: [yes]
Checkstyle issues: [no]*
ChangeLog updated: [yes]
Fixes the following jira:
 - PIM-1957