**Is your feature request related to a problem? Please describe.**
Config listeners.ssl on Linux (using Docker compose):
`listeners.ssl {
    bind = "0.0.0.0:8883"
    keyfile = "/etc/certs/key.pem"
    certfile = "/etc/certs/cert.pem"
    cacertfile = "/etc/certs/cacert.pem"
    verify_peer = true
    fail_if_no_peer_cert = true
}`

Run `docker start`, it reports error:
`FATAL /nanomq/nanomq/apps/broker.c:1122: nng_listener_create tls`

**Describe the solution you'd like**
Found from another issue: https://github.com/nanomq/nanomq/issues/1577. 
This error may cause by cert level, but we can't change cert. Can nanomq switch cert verify level?


Can we switch listeners.ssl cert generate level?
This is a GitHub Issue
repo:nanomq
owner:emqx
Title : Can we switch listeners.ssl cert generate level?
Issue date:

--- start body ---
**Is your feature request related to a problem? Please describe.**
Config listeners.ssl on Linux (using Docker compose):
`listeners.ssl {
    bind = "0.0.0.0:8883"
    keyfile = "/etc/certs/key.pem"
    certfile = "/etc/certs/cert.pem"
    cacertfile = "/etc/certs/cacert.pem"
    verify_peer = true
    fail_if_no_peer_cert = true
}`

Run `docker start`, it reports error:
`FATAL /nanomq/nanomq/apps/broker.c:1122: nng_listener_create tls`

**Describe the solution you'd like**
Found from another issue: https://github.com/nanomq/nanomq/issues/1577. 
This error may cause by cert level, but we can't change cert. Can nanomq switch cert verify level?


--- end body ---
