Add is_ubuntu function

This replaces all of the [[ "$os_PACKAGE" = "deb" ]] tests, except when
those tests are before straight calls to dpkg.

Change-Id: I8a3ebf1b1bc5a55d736f9258d5ba1d24dabf04ea
diff --git a/stack.sh b/stack.sh
index 55eafa8..9428356 100755
--- a/stack.sh
+++ b/stack.sh
@@ -677,7 +677,7 @@
 
 # Install package requirements
 echo_summary "Installing package prerequisites"
-if [[ "$os_PACKAGE" = "deb" ]]; then
+if is_ubuntu; then
     install_package $(get_packages $FILES/apts)
 elif is_suse; then
     install_package $(get_packages $FILES/rpms-suse)
@@ -726,7 +726,7 @@
     if is_quantum_ovs_base_plugin "$Q_PLUGIN"; then
         # Install deps
         # FIXME add to ``files/apts/quantum``, but don't install if not needed!
-        if [[ "$os_PACKAGE" = "deb" ]]; then
+        if is_ubuntu; then
             kernel_version=`cat /proc/version | cut -d " " -f3`
             install_package make fakeroot dkms openvswitch-switch openvswitch-datapath-dkms linux-headers-$kernel_version
         else