Usea a bean reference for GrailsLiquibase to allow customisation by plugin users

[acanby]

Allow plugin users to override the definition of GrailsLiquibase instances created per data source

Resolves #169

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}