infrastructure: set root disk to 100GB

Without setting a size for the root disk, AWS defaults
to 8GB.  Let's set that to 100GB so we don't have to
keep adding partitions later on.

Change-Id: I1893396f1629db4ed84501cc4b520c410ba019d4
diff --git a/infra.tf b/infra.tf
index ab06c6b..200b5cb 100644
--- a/infra.tf
+++ b/infra.tf
@@ -34,4 +34,8 @@
         Name = "staging-ci"
         Environment = "staging"
     }
+    root_block_device {
+        volume_size = 100
+        volume_type = "gp2"
+    }
 }