MEDIUM: cli/ssl: configure ssl on server at runtime

MEDIUM: cli/ssl: configure ssl on server at runtime

in the context of a progressive backend migration, we want to be able to
activate SSL on outgoing connections to the server at runtime without
reloading.
This patch adds a `set server ssl` command; in order to allow that:

- add `srv_use_ssl` to `show servers state` command for compatibility,
  also update associated parsing
- when using default-server ssl setting, and `no-ssl` on server line,
  init SSL ctx without activating it
- when triggering ssl API, de/activate SSL connections as requested
- clean ongoing connections as it is done for addr/port changes, without
  checking prior server state

example config:

backend be_foo
  default-server ssl
  server srv0 127.0.0.1:6011 weight 1 no-ssl

show servers state:

  5 be_foo 1 srv0 127.0.0.1 2 0 1 1 15 1 0 4 0 0 0 0 - 6011 - -1

where srv0 can switch to ssl later during the runtime:

  set server be_foo/srv0 ssl on

  5 be_foo 1 srv0 127.0.0.1 2 0 1 1 15 1 0 4 0 0 0 0 - 6011 - 1

Also update existing tests and create a new one.

Signed-off-by: William Dauchy <wdauchy@gmail.com>