vmcheck/overlay: Bump ostree version

We need the new ostree for livefs. Also while I'm here, rework things so we only
do one HTTP connection by passing multiple URLs at once.

tests/vmcheck/overlay.sh

@@ -64,9 +64,11 @@ rm -f vmcheck/usr/etc/{.pwd.lock,passwd-,group-,shadow-,gshadow-,subuid-,subgid-
 rsync -rlv /var/roothome/sync/insttree/usr/ vmcheck/usr/
 # ✀✀✀ BEGIN hack for https://github.com/projectatomic/rpm-ostree/pull/642 ✀✀✀
 ostree admin unlock || true
-for url in https://kojipkgs.fedoraproject.org//packages/ostree/2017.2/3.fc25/x86_64/ostree-{,libs-,grub2-}2017.2-3.fc25.x86_64.rpm; do
-    curl -sSL -O $url
-done
+ver=2017.3
+release=2.fc25
+baseurl=https://kojipkgs.fedoraproject.org//packages/ostree/${ver}/${release}/x86_64/ostree-
+suffix=${ver}-${release}.x86_64.rpm
+curl -sS -LO ${baseurl}${suffix} -LO ${baseurl}libs-${suffix} -LO ${baseurl}grub2-${suffix}
 rpm -Uvh --force *.rpm
 for x in *.rpm; do
     rpm2cpio $x | (cd vmcheck && cpio -div)