Implements: Archive publisher allow-empty setting.

The archive publisher did not allow setting the allowEmptyArchive setting in the YAML.
This change adds configuring the setting if it is defined in the YAML file. If the value
is 'true' then it is set, else it is 'false'. The default Jenkins behavior is 'false' disallowing
builds to pass without storing the archived data.

Updated the doc string to add that there is a default value (false) which is also the default Jenkins behavior.
Cleand up the value assignment, and typed it to a str().

fixed pep8 violation

Addresses patchset 3 request to update the doc string to use the test directly.
Would like to have more discussion around the second point in patchset 3 around having
the new code add a line of XML.

Change-Id: I5e0c9ed7e8ea669a8d0e8f267ebb02d5f0b6ae73
Merge "Implements: Archive publisher allow-empty setting."
Add support to specify GitLab version.

Change-Id: Iae3746540cb692b19278fd0bedae48bf976700d5
Add support for parameters in pipeline publisher.

Change-Id: Ie36d7c6a9daab17adf93586f79b92be0ed267c35
Merge "Add support for parameters in pipeline publisher."
 Explain howto use maven's pre/postbuilders

They're configured like the other builders but one needed to check the
source to verify this until now.

Change-Id: I97400ad9c7cb0dfb89fe2757c4bedce639e7ea24
maven-deploy: don't require id

The id element is eg. not needed for file deployment so don't make it
mandatory.

Change-Id: Ide83b18cbab6010033c3bd8e8264595d1d2d5fa2
Merge " Explain howto use maven's pre/postbuilders"
Merge "maven-deploy: don't require id"