[3.4.x] Check ifvarclass exclusion for common class promises. Issue #1780

One code path for setting a class within a common bundle did not previously check for ifvarclass exclusion:
HashVariables -> CheckCommonClassPromise -> ExpandPromise -> KeepClassContextPromise

This patch adds the check within KeepClassContextPromise for common bundles.

See [Issue 1780](https://cfengine.com/dev/issues/1780)