Ensure root device lookup picks the exact match if it exists

- This handles a theoretical case where both `/dev/sda` and `/dev/sda1` are listed
  in block devices, we want to choose the exact match

[#134734789](https://www.pivotaltracker.com/story/show/134734789)

src/bosh_aws_cpi/lib/cloud/aws/block_device_manager.rb

@@ -154,13 +154,17 @@ def default_root_disk_mapping
 
     def root_device_name
       if @root_device_name
+        # covers two cases:
+        # 1. root and block device match exactly
+        # 2. root is a partition and block device is the entire device
+        #    e.g. root == /dev/sda1 and block device == /dev/sda
         block_device_to_override = (@ami_block_device_names || {}).find do |name|
-          # covers two cases:
-          # 1. root and block device match exactly
-          # 2. root is a partition and block device is the entire device
-          #    e.g. root == /dev/sda1 and block device == /dev/sda
+          @root_device_name == name
+        end
+        block_device_to_override ||= (@ami_block_device_names || {}).find do |name|
           @root_device_name.start_with?(name)
         end
+
         return block_device_to_override if block_device_to_override
       end
 

src/bosh_aws_cpi/spec/unit/instance_manager_spec.rb

@@ -82,7 +82,7 @@ module Bosh::AwsCloud
           stemcell_id => instance_double('AWS::EC2::Image',
             block_devices: block_devices,
             root_device_name: 'fake-image-root-device',
-            block_device_mappings: { 'fake-image-root-device': {} },
+            block_device_mappings: { 'fake-image-root-device' => {} },
             virtualization_type: :hvm
           )
         })