OCPBUGS-62790: Use separate fs for ostree checkout on live ISO

Installations using ABI/assisted with 16GiB of RAM on the bootstrap node
were failing with "no space left on device" during bootstrapping. The
live ISO environment uses a tmpfs mounted at /var that is sized at 50%
of available RAM. On systems with 16GiB of RAM, this provides only 8GiB
of tmpfs space.

At the beginning of the bootstrap process, node-image-pull.sh creates an
ostree checkout underneath /var/ostree-container. When this is added to
the regular disk space usage of the later parts of the bootstrap, the
peak tmpfs usage hits around 9.4GiB.

This fix creates a separate 4GiB tmpfs for /var/ostree-container, so
that it is not subject to the limits on the size of /var.

data/data/bootstrap/files/etc/systemd/system/node-image-pull.service

@@ -1,7 +1,8 @@
 [Unit]
 Description=Node Image Pull
+Wants=var-ostree\x2dcontainer.mount
 Requires=network.target NetworkManager.service
-After=network.target
+After=network.target var-ostree\x2dcontainer.mount
 
 [Service]
 Type=oneshot

data/data/bootstrap/files/etc/systemd/system/var-ostree\x2dcontainer.mount

@@ -0,0 +1,10 @@
+[Unit]
+Requires=run-ephemeral.mount
+After=run-ephemeral.mount
+ConditionPathExists=/run/ostree-live
+
+[Mount]
+What=tmpfs
+Where=/var/ostree-container
+Type=tmpfs
+Options=size=4G