core: support overriding environment

Useful for distro settings.

Change-Id: Ib619c87ac41374ef5007beb3d6de068fcfeb3ff5
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>
packaging: otopi-1.0.1

Change-Id: I3c657e5b39b09be7a474714ffa71acd95ebf7b14
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>
packaging: post otopi-1.0.1

Change-Id: I3f97cd80da3d916182950faea674283aefa327a8
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>