CI: Generate ssh key pair on the fly

There is litte reason to share an ssh key common to all CI jobs, so
generate one for each on the fly.

Also use plain-text cloud-init config instead of base64 for readability

tests/Makefile

@@ -1,8 +1,3 @@
-init-packet:
-	mkdir -p $(HOME)/.ssh
-	echo $(PACKET_VM_SSH_PRIVATE_KEY) | base64 -d > $(HOME)/.ssh/id_rsa
-	chmod 400 $(HOME)/.ssh/id_rsa
-
 create-tf:
 	./scripts/create-tf.sh
 
@@ -12,7 +7,7 @@ delete-tf:
 $(ANSIBLE_INVENTORY):
 	mkdir $@
 
-create-packet: init-packet | $(ANSIBLE_INVENTORY)
+create-packet: | $(ANSIBLE_INVENTORY)
 	ansible-playbook cloud_playbooks/create-packet.yml -c local \
 		-e @"files/${CI_JOB_NAME}.yml"