Add manifest entry for dangerous grade images (new) (#1541)

* Add manifest to check grade status of an image

In our testing lab, we need to provision some devices with Ubuntu Core
images that are set to use the 'dangerous' grade.

This will fail the `image/model-grade` test.

Add a new `dangerous_grade_core_image` manifest to decide whether or not
we should be running this test.

It will be run by default (when there is no manifest entry, its value is
set to 'False'), but by setting it to True, the test will be skipped.

* Modify manifest name

Fix #1479

providers/base/units/image/jobs.pxu

@@ -78,8 +78,10 @@ flags: preserve-locale
 id: image/model-grade
 category_id: image
 _summary: Check that the model grade is correctly set
+imports: from com.canonical.plainbox import manifest
 requires:
  lsb.distributor_id == "Ubuntu Core" and int(lsb.release) >= 20
+ manifest.dangerous_grade_core_image == 'False'
 _purpose:
  Images with the 'dangerous' grade (the lowest of all available grades)
  results in certain security measures being relaxed.

providers/base/units/image/manifest.pxu

@@ -0,0 +1,5 @@
+unit: manifest entry
+id: dangerous_grade_core_image
+_name: Image is using 'dangerous' grade (should be set to 'No' unless you're doing SRU testing)
+_prompt: Does this setup have the following configuration?
+value-type: bool