Better config backwards compatibility

Previous commits added extra valid keys for the config file but just
assumed that those keys would always be present.  We will now check
to see if those keys exist, and if not we will add default values.

acmeproxy.pl

@@ -48,6 +48,15 @@
 # Early sanity checks
 die("acme dnslib provider not found: $config->{dns_provider}\n")
   unless (-f "$acme_home/dnsapi/$config->{dns_provider}.sh");
+if (!exists($config->{acmesh_extra_params_install})) {
+  $config->{acmesh_extra_params_install} = [];
+}
+if (!exists($config->{acmesh_extra_params_install_cert})) {
+  $config->{acmesh_extra_params_install_cert} = [];
+}
+if (!exists($config->{acmesh_extra_params_issue})) {
+  $config->{acmesh_extra_params_issue} = [];
+}
 foreach my $auth (@{$config->{auth}}) {
   if (exists($auth->{pass})) {
     logg "One or more users are defined with plaintext passwords. You should convert them to bcrypt hashes!";