Add field tls_versions to pools for restricing TLS versions used.
This is a colon-separated string of versions to be used.
Available values (as defined in octavia-lib):
SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
Add default_pool_tls_versions in octavia.conf
Note: TLSv1.3 connections will use haproxy's default ciphers
instead of the listener's tls_ciphers field
Change-Id: I480b7fb9756d98ba9dbcdfd1d4b193ce6868e291
Story: 2006733
Task: 37173
Depends-On: Ic33d9b9a256490ae1b048cdfd2475d6340509fdb
9a6da86481