Add sudo for install and zerotier-cli commands

[finchd]

The role as of v1.1.0 or commit:89c5635 (master) didn't work for me on Ubuntu 18.04 Bionic, every apt and zerotier-cli step needed become: yes.

This PR adds become: yes to the main.yml where install and join_network tasklists get included as all of their tasks require sudo, and adds one use in authorize_node.yml because of one usage of the zerotier-cli.

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[m4rcu5nl]

Hi. Thanks for the PR but this is more or less by design. The idea is to leave environment specific choices as much as possible in the playbook. The role only specifically drops privileges to do the api requests as the local user. This makes it possible to include it in your playbook as follows:

    roles:
         - { role: m4rcu5nl.zerotier, become: true }

I've updated the example in the README to reflect this.