Pipeline template for capella addons

[felixdo]

This makes standard addon Jenkinsfiles very simple:

capellaAddon {
url = <my addon git url>
name = 'MyAddon'
targetPlatform = "MyAddon/my/targetplatform/pom.xml"
versionFolder = 'some-custom-folder'
}

Optional parameters are:

targetPlatform:
if not present, the build will use a default addon targetplatform

versionFolder:
defaults to the name of the branch being built. can be used
to redirect master branch artifacts to the upcoming capella version
(see below)

The 'name' parameter will be used to for 3 things:

dropins zip filename
update site zip filename
download path segment for artifacts (see below)

Deployment

It is expected that the build produces a single dropins
zip and a single update site + update site zip. These
zipfiles are located by searching for category.xml.

The artifacts (updatesite, updatesite zip, dropins zip)
are then copied to

download.eclipse.org/capella/addons/<name>/zips/nightly/<versionFolder>
for update site and dropin zips.

download.eclipse.org/capella/addons/<name>/updates/nightly/<versionFolder>/<site-version>

where <versionFolder> defaults to the branch name. On master branches
this should be overwritten in Jenkinsfile, using the capella version
that the master branch currently targets.

[felixdo]

better read the real commit message, some of the formatting messed up in the pr page..