SonarQube/Git: Branch Name works only in multi-branch pipelines

[schnatterer]

env.BRANCH_NAME is only present in multi-branch pipeline builds, regular pipelines don't have information about branches.

Affected methods:

• Git.getBranchName()
• SonarQube.initMavenForRegularAnalysis()

We could use git rev-parse --abbrev-ref HEAD in Git class, instead. SonarQube could use Git class. Keeps everything DRY.

When fixed, update README.