451279 Do not add mvn descriptors in features

_When packaging a feature, and the archiver parameter is not specified
in the pom, set the addMavenDescriptor flag in the
MavenArchiveConfiguration to false. The user can overwrite this setting
using the <archive><addMavenDescriptor> config parameter.
_For packaging a plugin, the default is still to include the descriptor
added tests for that.
_Adding site doc for the archive parameter and how to change the
addMavenDescriptor flag.
_Adding 2 tests, which are testing the default and overwrite scenarios.

Bug: 451279
Change-Id: I1c4a2ae5196bc02d764f9f62b95d782ddf77f1d9
Signed-off-by: Martin Schreiber <m.schreiber@bachmann.info>