Add option to specify group for non-root install

For non-root install, it would be good if we could specify the group to install under. In current versions the same name for group as for the user is assumed in post-install script:

$SUDO chown $user:$user $installdir/etc/Columnstore.xml

While this is OK for users like root or mysql (and typical in many Linux versions to create a separate group with the same name for each user, by default), option to use some separate explicitly created and specified group (like "wheel" in FreeBSD case, or some dedicated group for users with SSH access allowed) may be useful sometimes.