Change the AWS default image volume-type from gp2 to gp3

gp3 is better in almost every way (cheaper, faster, more configurable).
It seems difficult to find a situation where gp2 would be a better
choice, so update the default when creating images to use gp3.

There are two locations where we can specify volume-type: image creation
(where the volume type becomes the default type for the image) and
instance creation (where we can override what the image specifies).
This change updates only the first (image creation), but not the second,
which has no default (which means to use whatever the image specified).

https://aws.amazon.com/ebs/general-purpose/

Change-Id: Ibfc5dfd3958e5b7dbd73c26584d6a5b8d3a1b4eb

doc/source/aws.rst

@@ -415,7 +415,7 @@ Selecting the ``aws`` driver adds the following options to the
 
       .. attr:: volume-type
          :type: str
-         :default: gp2
+         :default: gp3
 
          The root `EBS volume type`_ for the image.
          Only used with the

nodepool/driver/aws/config.py

@@ -103,7 +103,7 @@ class AwsProviderDiskImage(ConfigValue):
         self.architecture = image.get('architecture', 'x86_64')
         self.ena_support = image.get('ena-support', True)
         self.volume_size = image.get('volume-size', None)
-        self.volume_type = image.get('volume-type', 'gp2')
+        self.volume_type = image.get('volume-type', 'gp3')
         self.import_method = image.get('import-method', 'snapshot')
         self.iops = image.get('iops', None)
         self.throughput = image.get('throughput', None)

releasenotes/notes/gp3-default-63fd6eaba7f32011.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    The default :attr:`providers.[aws].diskimages.volume-type` for
+    AWS diskimages has been changed from `gp2` to `gp3`.