core: create prop in frontend pom for engine core

Create a property "engine.core.groupId" in frontend pom
and use it in all inheriting poms instead of using hardcoded string for
group id "org.ovirt.engine.core"
The change allows more clear use(reuse)  of a package name and will
also allow more easy refactoring/renaming if such occurs in the future
Similar to using engine.version constant for example...

Change-Id: I06b4e2886196ae4fdb7580033a83ec651adf1b11
Signed-off-by: Alissa Bonas <abonas@redhat.com>