containers: check item.container before pull/save

In later versions of Kubespray, certain items (such as Helm) are
no longer fetched as a container. This change checks that the
download manifest in Kubespray specifies that the item in question
is meant to be treated as a container before pulling and saving.

Signed-off-by: Cian Johnston <cian.johnston@est.tech>
Change-Id: Iedbfb6f020c4d89e050f1fa83209634affe3ebab