add parameters to deploy or not non snapshots or release (-DskipRelease or -DskipSnapshot)

maven-release-plugin creates a commit where the pom is not a SNAPSHOT version: if CI system is configured to build on every commit and to deploy the artifacts on every build (with the intent in general to deploy updated SNAPSHOTs on every commit), the CI will deploy the non-SNAPSHOT that has already been deployed by the user who launched mvn release:perform, then it will generate an error (trying to override existing release artifact) and/or create new staging repository and/or override artifacts partially (if repository not configured to block overrides)

we can add a new parameter -DdeployOnlySnaphots=true/false: it will be false per default for normal use, but can be configured to true in users' CI system.