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/swift b/lib/swift
index 366c467..140e5e9 100644
--- a/lib/swift
+++ b/lib/swift
@@ -159,7 +159,7 @@
         s,%SWIFT_DATA_DIR%,$SWIFT_DATA_DIR,;
     " $FILES/swift/rsyncd.conf | sudo tee /etc/rsyncd.conf
     # rsyncd.conf just prepared for 4 nodes
-    if [[ "$os_PACKAGE" = "deb" ]]; then
+    if is_ubuntu; then
         sudo sed -i '/^RSYNC_ENABLE=false/ { s/false/true/ }' /etc/default/rsync
     else
         sudo sed -i '/disable *= *yes/ { s/yes/no/ }' /etc/xinetd.d/rsync
@@ -341,7 +341,7 @@
     # (re)start rsyslog
     restart_service rsyslog
     # Start rsync
-    if [[ "$os_PACKAGE" = "deb" ]]; then
+    if is_ubuntu; then
         sudo /etc/init.d/rsync restart || :
     else
         sudo systemctl start xinetd.service