feat: add LXC container support for local chatmail development

Add cmdeploy "lxc-test" command to run cmdeploy against local containers,
with supplementary lxc-start, lxc-stop and lxc-status subcommands.
See doc/source/lxc.rst for full documentation including prerequisites,
DNS setup, TLS handling, DNS-free testing, and known limitations.

Apart from adding lxc-specific docs, tests, and implementation files in the cmdeploy/lxc directory,
this PR adds the --ssh-config option to cmdeploy run/dns/status/test commands and pyinfra invocations,
and also to sshexec (Execnet) handling.  This allows for the host to need no DNS entries for a relay,
and route all resolution through ssh-config.  This is used by the "lxc-test" command, which performs
a completely local setup -- again, see docs for more details.

While working on DNS/SSH things i also unified all zone-file handling
to use actual BIND format as it is easy enough to parse back.

doc/source/index.rst

@@ -16,5 +16,6 @@ Contributions and feedback welcome through the https://github.com/chatmail/relay
     proxy
     migrate
     overview
+    lxc
     related
     faq