Only warn about pam if it's used

On upgrading to the most recent master here, I got complaints like this:
```
Warning: Scope(Openvpn::Client[user1@vpn.clanspum.net]): Using $pam is deprecated. Use $authuserpass instead!
Warning: Scope(Openvpn::Client[user2@vpn.clanspum.net]): Using $pam is deprecated. Use $authuserpass instead!
Warning: Scope(Openvpn::Client[user3@vpn.clanspum.net]): Using $pam is deprecated. Use $authuserpass instead!
Warning: Scope(Openvpn::Client[user4@vpn.clanspum.net]): Using $pam is deprecated. Use $authuserpass instead!
```
I never used `$pam`, so I was surprised.  I added a little logic around it to only warn if `$pam` is set to something.