Add custom zookeeper_id for each host based in inventory

[gbaufake]

• Modifies the zookeeper_id on a template to allow hostvar if it is defined. If it is not specified, it uses the standard indexing from the template.
• Closes https://github.com/ansible-middleware/amq_streams/issues/81

[rmarting]

@gbaufake Just a comment to improve this PR (before a better revision of @rpelisse):

• The variables for a role should start with the name of the role (e.g: amq_streams_zookeeper_XXX)
• It is important to document the variable, default values, and scope to help its use (adding an example is much appreciated)

Thanks your contribution!