OGM-339 Skip integration test modules when 'skipITs' property  is used

JIRA: https://hibernate.atlassian.net/browse/OGM-339

In the integration tests sub-modules the build is always skipping the installation of the test sources. This cause failures when in the jar with tje tests sources is not in the local repository.

I've also change the name of the property from 'skipTests' to 'skipITs'. This way it is possible to run unit tests without running and build the integration tests.