This PR makes deployAtEnd work as expected even
if maven-deploy-plugin is not used as extension.

How it works: it uses mojo Context to store "markers" (and params):
* presence of marker means project was "processed"
* value of marker tells what should be done (true = deploy, false = skipped)
* if needed, other params are stored as well

UTs adjusted to provide plugin context (was null before).

Also, be clearer re intent. Still, cannot use enum directly, as
due classloading, there is no single enum.