Ansible Node - Enforce private key permission and bind permissions to scylla user

[fee-mendes]

When the source playbook is executed with become: True its relevant TLS certificates will be owned by root. This causes a problem, because later on when trying to copy we are unable to read the resulting private key file, as it is (correctly) created by default with mode 0600.

This commit let Ansible generate each PKI component with mode 0644. Then, ensure these are copied to the remote machine's with strict permissions to scylla user/group. Finally, ensure that the private key is only readable by its target user - both locally and remotely.

Fixes #139