Fix failing to set /etc/zookeeper/conf/myid when MYID is a Fixnum

In my case I am using numbers (Fixnum) in my $hosts parameter to the zookeeper class:

```
 class { 'zookeeper': 
        hosts    => hash(flatten(zip($zookeeper_hosts, range('1', size($zookeeper_hosts))))), # This generates a hash with FQDNs as keys and their 1-indexed position in the $zookeeper_hosts array as value.
        data_dir => '/var/lib/zookeeper', 
    } 
```

Instead of something less complicated with the values of the $hosts hash being Strings

```
 class { 'zookeeper': 
        hosts    => { 'hostname-1' => '1', 'hostname-2' => '2'},
        data_dir => '/var/lib/zookeeper', 
    } 
```

The following error occurs when using numbers (the first code block) instead of strings (the second code block):

```
err: Failed to apply catalog: Parameter content failed: Munging failed for value <myid value> in class content: can't convert Fixnum into String
```

Simply converting the $myid variable to a string as file contents fixes this problem.