maas-region: option to always use GPT

MAAS uses MBR for boot disks smaller than 2 TiB. This change provides an
option to force the use of GPT, regardless of boot disk size. The chart
value is: conf.maas.force_gpt=true.

The 2 TiB "threshold" for when GPT is required is simply lowered to 0:
https://github.com/maas/maas/blob/2.3/src/maasserver/models/partitiontable.py#L51-L53

This change could be accomplished with a patch to the maas-region image
directly, but then it would not be configurable, and it may not be
useful for all users. Using sed in the startup script seems like a fair
solution.

Change-Id: I87d3f4b9c97048cdef383cbd15c5a16ac219066b

charts/maas/templates/bin/_start.sh.tpl

@@ -52,5 +52,10 @@ done
 if [[ $sh_set = false ]]; then
   exit 1
 fi
+{{- if .Values.conf.maas.force_gpt }}
+# Forcing the use of GPT irrespective of boot disk size
+# https://github.com/maas/maas/blob/2.3/src/maasserver/models/partitiontable.py#L51-L53
+sed -i '/^GPT_REQUIRED_SIZE =/c\GPT_REQUIRED_SIZE = 0' /usr/lib/python3/dist-packages/maasserver/models/partitiontable.py
+{{- end }}
 set -e
 exec /sbin/init --log-target=console 3>&1

charts/maas/values.yaml

@@ -235,6 +235,12 @@ conf:
       secret:
         namespace: maas
         name: maas-api-key
+    # By default, MAAS will use MBR for boot disks smaller than 2 TiB.
+    # Set force_gpt: true to always use GPT.
+    # NOTE: This is not a standard MAAS setting, and enabling it will cause
+    # modification of a file during maas-region pod startup:
+    # /usr/lib/python3/dist-packages/maasserver/models/partitiontable.py
+    force_gpt: false
     extra_settings:
       # Additional settings available via maas $PROFILE maas set-config
       # Marks if the initial intro has been completed: true or false