[MRELEASE-1077] Support for specifying profiles that can be activated during the prepare phase.

[nielsbasjes]

In some of my projects I have the website for the project as part of the code base. In those situations I want the website to show the latest released version of the project (I.e. not the current SNAPSHOT that is in the current code base).

To allow updating this version in my website automatically as part of the release process; I would like to have a way to activate the relevant plugins only during the prepare phase of the new release so that I can run the appropriate scripts to change the website.

The implementation is incredibly simple: I only had to attach the getAdditionalProfiles() for the PrepareReleaseMojo to a property. Most of the code is a new integration test.

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