Merge infra and k3 into one directory again

Since I don't have multiple terraform steps anymore it simply doesn't
make sense to me anymore to split all tasks into separate folders.
Instead I try to be as clear as possible in the README to make it easy
to follow the structure in the future without too much headache.

inventory.ini.tftpl

@@ -0,0 +1,20 @@
+[server]
+%{for ip in server_ips~}
+${ip}
+%{endfor~}
+
+[server:vars]
+ansible_port=${ssh_port}
+
+[agent]
+%{for ip in agent_ips~}
+${ip}
+%{endfor~}
+
+[k3s_cluster:children]
+server
+agent
+
+[k3s_cluster:vars]
+network_cidr=${network_cidr}
+private_nat=${private_nat}