invoke 'dist-upgrade' instead of 'upgrade' on for upgrades.

In general, dist-upgrade is the correct behavior here.
It will get a new kernel, though, which could be annoying.  So, allow
a way to turn it off (by setting  'apt_get_upgrade_subcommand: upgrade').

cloudinit/distros/debian.py

@@ -161,7 +161,13 @@ class Distro(distros.Distro):
         elif args and isinstance(args, list):
             cmd.extend(args)
 
-        cmd.append(command)
+        
+        subcmd = command
+        if command == "upgrade":
+            subcmd = self.get_option("apt_get_upgrade_subcommand",
+                                     "dist-upgrade")
+
+        cmd.append(subcmd)
 
         pkglist = util.expand_package_list('%s=%s', pkgs)
         cmd.extend(pkglist)

doc/examples/cloud-config.txt

@@ -125,6 +125,19 @@ apt_sources:
       =Y2oI
       -----END PGP PUBLIC KEY BLOCK-----
 
+# apt_get_command: [command, argument, argument]
+# Specify a different 'apt-get' command. must be a list.  subcommands are
+# appended to it. default is:
+# ['apt-get', '--option=Dpkg::Options::=--force-confold',
+#  '--option=Dpkg::options::=--force-unsafe-io', '--assume-yes', '--quiet']
+#
+# apt_get_upgrade_subcommand:
+# Specify a different 'apt-get upgrade' subcommand.  when 'apt_upgrade' or
+# package_upgrade is set to true above, then this subcommand will be invoked.
+# default is 'dist-upgrade'.  For example, you could set this to 'upgrade'.
+apt_get_upgrade_subcommand: dist-upgrade
+
+
 # Install additional packages on first boot
 #
 # Default: none