Jenkinsfile: Allow specifying packaging targets.

The default is to build all the packaging targets (minus the *_qt
and *_arm ones).  Add a PACKAGING_TARGETS parameter that allows
specifying which targets to build.

* Jenkinsfile: (PACKAGING_TARGETS): New parameter.
('Build packages'): Use it unless it's the empty string.

GitLab: jami-packaging#98
Change-Id: Idf1b4460a0edac7cbe6a82b9db1d204af78de801