bugfix: Ensure kernel_package param is not required

A previous patchset introduced a new kernel
param option 'kernel_package.' This patch corrects
the logic in that so that the parameter is not a
required parameter - and if absent falls back to
the traditional MaaS behavior which will select the
latest kernel from the appropriate line.

Change-Id: Icc62b27e0f39914fb73fb9f655d9b7b0b6c6f489

images/maas-region-controller/2.3_kernel_package.patch

@@ -1,12 +1,15 @@
 --- preseed.py	2018-10-15 12:52:41.611027184 -0700
-+++ preseed2.py	2018-10-16 13:42:51.824755746 -0700
-@@ -317,7 +317,23 @@
++++ preseed2.py	2018-10-19 07:26:10.985282758 -0700
+@@ -317,7 +317,25 @@
      The BootResourceFile table contains a mapping between hwe kernels and
      Ubuntu package names. If this mapping is missing we fall back to letting
      Curtin figure out which kernel should be installed"""
--    kpackage = BootResource.objects.get_kpackage_for_node(node)
++
++    # previous logic to retrieve kpackage parameter
+     kpackage = BootResource.objects.get_kpackage_for_node(node)
 +
 +    # determine if this node has kernel parameters applied by drydock
++    # and override kpackage if we discover the right properties
 +    kernel_opt_tag = "%s_kp" % (node.hostname)
 +    if kernel_opt_tag in node.tag_names():
 +
@@ -19,8 +22,6 @@
 +        kdict = dict(kparam.split('=') for kparam in kparams)
 +        if 'kernel_package' in kdict:
 +            kpackage = kdict['kernel_package']
-+    else:
-+        kpackage = BootResource.objects.get_kpackage_for_node(node)
 +
      if kpackage:
          kernel_config = {