core: support overriding environment
core: support overriding environment

Useful for distro settings.

Change-Id: I933373b31c3f2e1b8f7a8a5624b47eaa8b0a70e4
Signed-off-by: Alon Bar-Lev <2e3ceb4d37adc6ae3ccc00d3646e0e0a036ace03@redhat.com>

oVirt/ovirt-host-deploy,alonbl/ovirt-host-deploy,alonbl/ovirt-host-deploy,oVirt/ovirt-host-deploy,oVirt/ovirt-host-deploy