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/lib/horizon b/lib/horizon
index 189ca10..d0a0f05 100644
--- a/lib/horizon
+++ b/lib/horizon
@@ -71,7 +71,7 @@
     sudo mkdir -p $HORIZON_DIR/.blackhole
 
 
-    if [[ "$os_PACKAGE" = "deb" ]]; then
+    if is_ubuntu; then
         APACHE_NAME=apache2
         APACHE_CONF=sites-available/horizon
         # Clean up the old config name
@@ -110,7 +110,7 @@
 # install_horizon() - Collect source and prepare
 function install_horizon() {
     # Apache installation, because we mark it NOPRIME
-    if [[ "$os_PACKAGE" = "deb" ]]; then
+    if is_ubuntu; then
         # Install apache2, which is NOPRIME'd
         install_package apache2 libapache2-mod-wsgi
     elif is_suse; then
@@ -121,7 +121,7 @@
     fi
 
     # NOTE(sdague) quantal changed the name of the node binary
-    if [[ "$os_PACKAGE" = "deb" ]]; then
+    if is_ubuntu; then
         if [[ ! -e "/usr/bin/node" ]]; then
             install_package nodejs-legacy
         fi