Ensure hosts are destroyed if post-suite fails

When running `rake ci:test:aio[true]`, if the post-suite failed and we're not
preserving hosts, then the `beaker destroy` command would be skipped. Use a
nested ensure so the failure of post-suite doesn't prevent `beaker destroy` from
being called.

The nested ensure also guarantees that the `post-suite` will be called even if
the `pre-suite` fails, which can happen if the agent fails to install. This
matches the behavior for the "non-retry" version of the `ci:test:aio` rake task.

```
$ git diff
diff --git a/acceptance/teardown/common/099_Archive_Logs.rb b/acceptance/teardown/common/099_Archive_Logs.rb
index d940e2bea..2302633e1 100644
--- a/acceptance/teardown/common/099_Archive_Logs.rb
+++ b/acceptance/teardown/common/099_Archive_Logs.rb
@@ -1,5 +1,7 @@
 require 'date'

+raise "whoops"
+
 def file_glob(host, path)
   result = on(host, "ls #{path}", :acceptable_exit_codes => [0, 2])
   return [] if result.exit_code != 0

$ export BEAKER_PUPPET_VERSION=/home/josh/work/beaker-puppet
$ bundle update
...
$ bundle exec rake 'ci:test:aio[true]' SHA=8.4.0 TESTS=tests/load_libfacter.rb HOSTS=hosts.yaml
...
Begin teardown/common/099_Archive_Logs.rb
RuntimeError: whoops
...
beaker destroy
```

tasks/ci.rake

@@ -334,23 +334,26 @@ def beaker_suite_retry(type)
   beaker(:provision)
 
   begin
-    beaker(:exec, 'pre-suite', '--preserve-state', '--pre-suite', pre_suites(type))
-    beaker(:exec, 'pre-suite', '--preserve-state')
-
     begin
-      json_results_file = Tempfile.new
-      beaker(:exec, ENV.fetch('TESTS', nil), '--test-results-file', json_results_file.path)
-    rescue RuntimeError => e
-      puts "ERROR: #{e.message}"
-      tests_to_rerun = JSON.load(File.read(json_results_file.path))
-      raise e if tests_to_rerun.nil? || tests_to_rerun.empty?
-
-      puts '*** Retrying the following:'
-      puts tests_to_rerun.map { |spec| "  #{spec}" }
-      beaker(:exec, tests_to_rerun.map { |str| "#{str}" }.join(','))
+      beaker(:exec, 'pre-suite', '--preserve-state', '--pre-suite', pre_suites(type))
+      beaker(:exec, 'pre-suite', '--preserve-state')
+
+      begin
+        json_results_file = Tempfile.new
+        beaker(:exec, ENV.fetch('TESTS', nil), '--test-results-file', json_results_file.path)
+      rescue RuntimeError => e
+        puts "ERROR: #{e.message}"
+        tests_to_rerun = JSON.load(File.read(json_results_file.path))
+        raise e if tests_to_rerun.nil? || tests_to_rerun.empty?
+
+        puts '*** Retrying the following:'
+        puts tests_to_rerun.map { |spec| "  #{spec}" }
+        beaker(:exec, tests_to_rerun.map { |str| "#{str}" }.join(','))
+      end
+    ensure
+      beaker(:exec, 'post-suite')
     end
   ensure
-    beaker(:exec, 'post-suite')
     preserve_hosts = ENV['OPTIONS'].include?('--preserve-hosts=always') if ENV['OPTIONS']
     beaker(:destroy) unless preserve_hosts
   end