Missing validation of altDeploymentRepository mojo parameter

Reproducer: https://github.com/cstamas/mvn-md-bug

Command like: mvn clean deploy -DaltDeploymentRepository=localhost-nexus::default::http://localhost:8081/content/repositories/snapshots/ (w/ 3.0.0-M1 m-d-p, needs POM edit)

As seen in MDEPLOY-274, m-d-p 2.x succeeds, while 3.x fails with same CLI command line.

True, the parameter did change between 2.x and 3.x, still IMO validation is missing: 3.x should either fail early (not at attempted deploy w/o credentials) or at least WARN about "not recognized parameter", or best. accept 2.x format as well, but only if layout is "default".

Maybe some other changed parameters needs some validation as well.