myid fix

after the setup my `myid` file was empty. I found a couple of problems with it:
1. it should start with [1](http://stackoverflow.com/a/11498597/727938)
2. the same problem as in [pull request #3](https://github.com/geoforce/chef-zookeeper/pull/3): `n[:zookeper][:ipaddress]`
3. for some setups, you don't really know your external ip: e.g. vagrant boxes or aws instances. So I added an ability to mandatory set `node[:zookeper][:myid]` attribute.