harden haproxy TLS configuration

harden the TLS default config according to the mozilla
"modern" recommendation:

https://ssl-config.mozilla.org/#server=haproxy&version=2.1&config=modern&openssl=1.1.1k&guideline=5.7



if you want to revert to the old settings, set:

kolla_haproxy_ssl_settings: "legacy" in globals.yaml
alternatively you can also set it to "intermediate"
for a middle ground between security and accessibility.

this also adjusts the glance and neutron tls proxy ssl settings
in their dedicated haproxy config templates to use the same mechanism.

also add some haproxy related docs to the TLS guide and cross reference
it from the haproxy-guide.

Closes-Bug: #2060787

Signed-off-by: Sven Kieske <kieske@osism.tech>
Change-Id: I311c374b34f22c78cc5bcf91e5ce3924c62568b6