Add skipClean in webui build to support building it as an addon

When building nuxeo, Web UI is built as an addon. This means a single mvn clean deploy in nuxeo will trigger the same on every addons.
When making a release the bower references have to be set upfront to align them with the elements repositories.

Currently the skipInstall option is useful for this and is used in the webui pipeline but the clean target from maven deletes the bower_components folder containing the aligned elements.
To support the build during a release, adding a skipClean option would allow to keep the aligned elements.