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.
* cleanup: remove CFFI deltachat bindings usage, and consolidate test support with rpc-bindings
major simplification: all chatmail fixtures used in the test are now created inside the cmdeploy plugin,
and do not inherit anything from other fixture machineries, let alone the legacy deltachat CFFI ones.
also fix that pytest report headers show correct chatmail domains under test
