Alter integration tests for jcasgen-maven-plugin to skip m2e tests

The m2e test runs a build using Tycho, with a "temporary" new maven repository, that starts out essentially empty. This build requires a large amount of network traffic against various hosts, including the eclipse p2 sites, for downloading the artifacts needed to populate the maven temporary repo. Because of this, the m2e build takes almost 10 minutes (on my laptop with large network connectivity bandwith - not sure how it would on Jenkins machines). It is good to run this test once when jcasgen-maven-plugin is updated. But it is better to disable it normally.

Disable it in a manner where it can be easily re-enabled - by adding a configuration to the invoker plugin.