Coin should allow "current branch" variable in module_config.yaml

We allow human errors when using module_config.yaml to define licence chek instructions trough module_config.yaml.

For example module in 5.14 has following step in licence_check instructions:

 - type: InstallSourceArchive
 maxTimeInSeconds: 600
 maxTimeBetweenOutput: 600
 project: qt/qtbase
 ref: 5.14
 directory: qt/qtbase
 userMessageOnFailure: >
 Could not install source archive. Please investigate why.

Which is wrong once for example 5.14.1 is created, while then the qtbase ref should be 5.14.1.
We should allow defining something similar to:

 - type: InstallSourceArchive
 maxTimeInSeconds: 600
 maxTimeBetweenOutput: 600
 project: qt/qtbase
 ref: \{{.CurrentBranch}}
 directory: qt/qtbase
 userMessageOnFailure: >
 Could not install source archive. Please investigate why.

To pass the check one has to extract qtbase source tar to get correct reference headers. For that one should pass the ref identifying which branch we should take it. Usually correct headers are in same qtbase branch as what we are checking. At the moment ref can only be a valid branch. This introduces possible error situation when branching/merging and one forgot to manually update the qtbase ref to match the new branch. To fix this, we should allow symbolic ref which would be mapped to same branch as where the instruction is defined.