Jenkinsfile: Add the ability to opt out of updating submodules.

* Jenkinsfile (submodules): New variable.
[parameters]{WITH_MANUAL_SUBMODULES}: New parameter.
(Fetch submodules): Adapt the message to the parameter's value.  Add
the '--remote' option to 'git submodule' unless WITH_MANUAL_SUBMODULES
is true.

GitLab: jami-packaging#98
Change-Id: Ie5f0521bed16303e751e8e19174f18a90bccefcc