Replate non-portable echo -e with printf

POSIX does not define `-e` as a valid option for echo(1).  Rely on
printf(1) instead.

This fix acceptance tests on Ubuntu ARM where echo(1) outputs `-e`
causing an invalid configuration:

```
/home/testuser-ssh/.ssh/config: line 1: Bad configuration option: -e
```

Author: smortex

spec/acceptance/clone_repo_spec.rb

@@ -453,7 +453,7 @@
 
       # copy public key to authorized_keys
       run_shell('cat /home/testuser-ssh/.ssh/id_rsa.pub > /home/testuser-ssh/.ssh/authorized_keys')
-      run_shell('echo -e "Host localhost\n\tStrictHostKeyChecking no\n" > /home/testuser-ssh/.ssh/config')
+      run_shell('printf "Host localhost\n\tStrictHostKeyChecking no\n" > /home/testuser-ssh/.ssh/config')
       run_shell('chown -R testuser-ssh:testuser-ssh /home/testuser-ssh/.ssh')
       run_shell('rm -rf /var/run/nologin')
     end