Refactor steps/vpn.py

This is pretty short file which was recently refactored. Slight changes, removed some sleeps, used nmci.pexpect.

Part of #62

@RunTests:openvpn_ipv4,openvpn_ipv4_neverdefault,openvpn_ipv6,openvpn_ipv6_neverdefault,openvpn_set_mtu,openvpn_set_firewall_zone,openvpn_terminate,openvpn_delete_active_connection,openvpn_persist,dns_resolved_two_default,dns_resolved_one_default,dns_resolved_two_default_with_priority,dns_resolved_two_default_with_negative_priority,dns_resolved_no_default,dns_resolved_full_tunnel_vpn,dns_resolved_split_tunnel_vpn,dns_resolved_add_remove_ipv6_dns,dns_priority,dns_priority_config,dns_default_two_default,dns_default_one_default,dns_default_two_default_with_priority,dns_default_two_default_with_negative_priority,dns_default_no_default,dns_default_full_tunnel_vpn,dns_default_split_tunnel_vpn,dns_default_split_tunnel_vpn_same_priority,dns_dnsmasq_two_default,dns_dnsmasq_one_default,dns_dnsmasq_two_default_with_priority,dns_dnsmasq_two_default_with_negative_priority,dns_dnsmasq_no_default,dns_dnsmasq_full_tunnel_vpn,dns_dnsmasq_split_tunnel_vpn,dns_dnsmasq_driver_removal,dns_dnsmasq_kill,dns_dnsmasq_kill_ratelimit,dns_multiple_options,dns_none,remove_dns_none,resolv_conf_dangling_symlink,resolv_conf_do_not_overwrite_symlink,resolv_conf_dns_priority,dns_openshift_dualstack_slow_v4,dns_openshift_dualstack_slow_v6,dns_openshift_v6_only_slow_v6,dns_dnsmasq_dnssec_opts,dns_resolved_dnssec_opts,dns_resolved_add_mdns,vpn_describe,multiple_vpn_connections,vpn_list_args,iptunnel_create_modify,iptunnel_restart,iptunnel_ip6gre_create_device,wireguard_activate_connection,vpnc_add_profile,vpnc_terminate,libreswan_ikev1_aggressive,libreswan_ikev1_main,libreswan_ikev2,libreswan_add_profile_wrong_password,libreswan_terminate,libreswan_delete_active_profile,libreswan_dns,libreswan_start_as_secondary,libreswan_connection_renewal,libreswan_provides_and_obsoletes,libreswan_import,libreswan_export,libreswan_autocompletion,libreswan_activate_asking_for_password,libreswan_activate_asking_for_password_with_delay,libreswan_activate_asking_for_password_and_secret,libreswan_reimport,libreswan_reimport_ikev2,libreswan_configurable_options_reimport,vpn_keep_username_from_data,strongswan_add_profile,strongswan_connection_renewal,strongswan_terminate,strongswan_delete_active_profile,strongswan_dns,strongswan_start_as_secondary,pptp_add_profile,pptp_passwd_file,pptp_terminate