core: do not allow execute process with non string argument or environment

python fails anyway, but we like better message.

Related-To: https://bugzilla.redhat.com/1155596
Change-Id: I8d79c4e29451779a0d98942de3bca1525a15c0ab
Signed-off-by: Yedidyah Bar David <didi@redhat.com>
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>