Allow use of sonar.groovy for pull requests (multibranch)

BRANCH_NAME
    For a multibranch project, this will be set to the name of the branch being built, for example in case you wish to deploy to production from master but not from feature branches.
CHANGE_ID
    For a multibranch project corresponding to some kind of change request, this will be set to the change ID, such as a pull request number.
CHANGE_URL
    For a multibranch project corresponding to some kind of change request, this will be set to the change URL.
CHANGE_TITLE
    For a multibranch project corresponding to some kind of change request, this will be set to the title of the change.
CHANGE_AUTHOR
    For a multibranch project corresponding to some kind of change request, this will be set to the username of the author of the proposed change.
CHANGE_AUTHOR_DISPLAY_NAME
    For a multibranch project corresponding to some kind of change request, this will be set to the human name of the author.
CHANGE_AUTHOR_EMAIL
    For a multibranch project corresponding to some kind of change request, this will be set to the email address of the author.
CHANGE_TARGET
    For a multibranch project corresponding to some kind of change request, this will be set to the target or base branch to which the change could be merged.