transport tls: use SSL_VERIFY_NONE by default

[daipom]

Which issue(s) this PR fixes:

• Fixes Syslog TLS: [client_cert_auth false] settings is not applied if [insecure true] is not set. #4462

What this PR does / why we need it:
Use SSL_VERIFY_NONE by default for transport tls.
(VERIFY_PEER is used by default now).

VERIFY_NONE should be used when client_cert_auth false (default).

Before this fix, we need to set insecure true for this.
However, insecure option should mainly be for cipher strength.
It would not be intended VERIFY_PEER without VERIFY_FAIL_IF_NO_PEER_CERT was used even if client_cert_auth false.
(When VERIFY_PEER without VERIFY_FAIL_IF_NO_PEER_CERT, server does certification only when clients send its certificate.
This would be why we overlooked it long time)

Before:

insecure	client_cert_auth	verify_mode
false	fales	VERIFY_PEER
false	true	VERIFY_PEER, VERIFY_FAIL_IF_NO_PEER_CERT
true	false	VERIFY_NONE
true	true	VERIFY_PEER, VERIFY_FAIL_IF_NO_PEER_CERT

After:

insecure	client_cert_auth	verify_mode
false	fales	VERIFY_NONE
false	true	VERIFY_PEER, VERIFY_FAIL_IF_NO_PEER_CERT
true	false	VERIFY_NONE
true	true	VERIFY_PEER, VERIFY_FAIL_IF_NO_PEER_CERT

Docs Changes:
Not needed.

Release Note:
The same as the title.

[kenhys]

LGTM.