Got this assertion:

    NetworkManager[12939]: <debug> [1536917977.4868] active-connection[0x563d8fd34540]: set state deactivated (was deactivating)
    ...
    NetworkManager[12939]: nm-openvpn[1106] <info>  openvpn[1132]: send SIGTERM
    NetworkManager[12939]: nm-openvpn[1106] <info>  wait for 1 openvpn processes to terminate...
    NetworkManager[12939]: nm-openvpn[1106] <warn>  openvpn[1132] exited with error code 1
    NetworkManager[12939]: <info>  [1536917977.5035] vpn-connection[0x563d8fd34540,2fdeaea3-975f-4325-8305-83ebca5eaa26,"my-openvpn-Red-Hat",0]: VPN plugin: requested secrets; state disconnected (9)
    NetworkManager[12939]: plugin_interactive_secrets_required: assertion 'priv->vpn_state == STATE_CONNECT || priv->vpn_state == STATE_NEED_AUTH' failed

Meaning. We should either ensure that secrets_required_cb() signal callback
is disconnected from proxy's signal, or we gracefully handle callbacks at
unexpected moments. Do the latter.

(cherry picked from commit 92344dd0)
(cherry picked from commit 011dd919)
(cherry picked from commit 0e633c23)