Fixing the P2 legacy IT merge.

The legacy-testsuite uses nexus-test-env-m-p that
in turn does NOT employ ArtifactHandlers to figure
out artifact properties from it's type (ie. nexus-plugin
is c:bundle and e:zip). Hence, it needs "raw" coordinates
to properly pick up the plugin it's about to install
to the Nx test subject.

plugins/pom now holds (basically duplicated) entry
for P2 plugin, but this should be sorted out by some
other means, probably by fixing n-t-e-m-p.

Verified locally, legacy ITs now executes properly,
before it was not even started due to goal
setup-enviornment failure, failing the build (as being
unable to "resolve" the p2 plugin from dependencies).