improve only_if condition

Signed-off-by: Martin Schurz <Martin.Schurz@t-systems.com>

Author: schurzi

controls/sshd_spec.rb

@@ -525,8 +525,8 @@
   impact 1.0
   title 'Server: RSA HostKey size'
   desc 'Verifies if RSA HostKey size >= 4096'
-  only_if('RSA HostKey exists') do
-    file("#{sshd_custom_hostkeys_path}/ssh_host_rsa_key").exists?
+  only_if('RSA HostKey is readable') do
+    File.readable?("#{sshd_custom_hostkeys_path}/ssh_host_rsa_key")
   end
 
   describe bash("test $(ssh-keygen -l -f #{sshd_custom_hostkeys_path}/ssh_host_rsa_key | awk '$1 < 4096 { print $1 }' | wc -l) -eq 0") do