agent: datadir as default home for the ovirtagent user

Solves the issue with starting the service when polyinstantiated
namespaces are used on the system.
Following the Fedora guideline:
    http://fedoraproject.org/wiki/Packaging:UsersAndGroups

Bug-Url: https://bugzilla.redhat.com/883124
Change-Id: I08cd6e8ca4341c62c0e5f18b6c6226a103873f62
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>