BUG-6652: generate features.xml with karaf-maven-plugin

BUG-6652: generate features.xml with karaf-maven-plugin

features.xml can be generated from POM information; this allows us to
produce features which are simultaneously Karaf artifacts and Maven
artifacts. Thus downstream projects can simply depend (at a Maven
level) on Karaf features — this automatically gives the correct Maven
dependencies at build and test time, and the correct Karaf feature
dependencies at run time.

Karaf 3.0.8 now includes enough features handling improvements to
produce useful results here. This patch preserves old features for
backwards-compatibility, which results in some inelegant feature
names; the intention is to allow experimentation in downstream
projects without breaking anything.

This patch introduces two new parent POMs: single-feature-parent is
intended for modules packaging a single feature, feature-repo-parent
for modules packaging a repository of features. SingleFeatureTest is
used to test the feature repositories — this ensures that the features
we ship are tested. single-feature-parent doesn't run
SingleFeatureTest to avoid testing features twice.

Change-Id: I2858891618360710a13756dbd4d005aa1a48c703
Signed-off-by: Stephen Kitt <skitt@redhat.com>