Some fixes, changes, and modularizing

[andyshinn]

This also supersedes #13. The biggest thing to note here is that it removes the use of deprecated include and uses import_tasks which is a Ansible 2.4+ feature. It would make sense to bump the major version of this on Ansible Galaxy if this is merged.

Some of the changes happening here:

• Joining networks and authorizing nodes are now separate actions. They run when zerotier_accesstoken or zerotier_network_id are set.
• Variables zerotier_accesstoken and zerotier_network_id are now optional and control whether a node is authorized or it is joined to a network respectively.
• In my setup I preferred to use the short name so I have added an option for zerotier_register_short_hostname. It defaults to false to retain the existing behavior.
• I changed the curl tasks to use the url module instead. This should negate the need for curl on the hosts.
• Modularized some of the tasks to use import_tasks (static) and include_tasks (dynamic) new directives of Ansible 2.4.