Fixes for custom datadir setting use case

When using the following attributes I needed this patch to get a successful chef-client run.

```ruby
set["percona"]["server"]["datadir"]          = "/data/mysql"
set["percona"]["conf"]["mysqld"]["datadir"]  = "/data/mysql"
set["percona"]["conf"]["mysqld"]["pid-file"] = "/data/mysql/mysqld.pid"
set["percona"]["conf"]["mysqld"]["socket"]   = "/data/mysql/mysqld.sock"
set["percona"]["conf"]["client"]["socket"]   = "/data/mysql/mysqld.sock"
```

I also tested without the above attributes as well to make sure it still worked as well.