config: add missing ssl.ssl_cert for etcd

etcd configuration section allows to connect to TLS-encrypted etcd
cluster, providing a way to pass `ssl.ssl_key`. But it is not enough
when etcd server have client cert auth enabled and has a CA file, since
it requires a ssl_cert as well. Actually, propagating ssl_cert is
already a part of the EE connect code [1], we just missing the top-level
config option.

Fixes https://github.com/tarantool/tarantool-ee/issues/827

1. https://github.com/tarantool/tarantool-ee/blame/1138443c46e7a6e1bb855277bc6cb3333240131c/src/box/lua/config/source/etcd.lua#L103

@TarantoolBot document
Title: config: add missing ssl.ssl_cert for etcd

etcd configuration section already allows to set `ssl.ssl_key`. Now it
also allows to pass `ssl.ssl_cert`.