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.

```
$ 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

@@ -347,9 +347,10 @@ def beaker_suite_retry(type)
       puts '*** Retrying the following:'
       puts tests_to_rerun.map { |spec| "  #{spec}" }
       beaker(:exec, tests_to_rerun.map { |str| "#{str}" }.join(','))
+    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