Use EPEL for debootstrap on centos

I747c2b8754effbc6ec82af3bf7543fd9599a6c14 removed the
centos-openstack-octata repos from the base image.  We were previously
getting debootstrap from there, so this broke the centos-7 based jobs.
Enable EPEL for the install so we use that version of debootstrap.

Additionally we noticed that dpkg wasn't installed on the build host
either, but is actually required (calls were silently failing ... a
problem for another day).  Add this.

Change-Id: I1e9db5130a05a004c3c877d1444d7097023c06e2

tests/install_test_deps.sh

@@ -17,8 +17,9 @@ sudo apt-get install -y --force-yes \
         rpm \
         uuid-runtime \
         yum-utils || \
-    sudo yum -y install \
+    sudo yum -y install --enablerepo=epel \
         bzip2 \
+        dpkg \
         debootstrap \
         docker \
         kpartx \