Adopt tempesta-tech.com in LXC container test (#602)

* Add LXC testing backend

* Add LXC container healthcheck

* Remove unused variables

* Remove reference to another branch

* Add LXC server factory

* Remove unused code

* LXC pretest snapshot

* Semi-working site test

* rework LXCServer:
- subprocess changed to `node.run_cmd`;
- the waiting moved from run_start to wait_for_connection;
- removed all optional functions for this class. They are not necessary, but it makes the code more difficult. We don't plan to create other containers or multiple ports.

* fix and add tests for tempesta-tech with LXCServer, update default test configs

* remove docker and tests using docker.

* add tempesta-tech as git submodule

* remove docker-compose backend, update README.

* typo fix and update tests_disabled_* files

* change link for tempesta-tech module to `https` because currently CI uses creds with login/password

* change IP for `cache_purge_acl` because by default the client IP in test framework is 127.0.0.2

* add test case
- add a new post page;
- add a new comment to this page;
- update cache;
- cache the comment is present on this page;

* fix setup.sh

* typo fix and update comment

---------

Co-authored-by: Roman <rb@tempesta-tech.com>

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "tempesta-tech.com"]
+	path = tempesta-tech.com
+	url = https://github.com/tempesta-tech/tempesta-tech.com.git

README.md

@@ -43,7 +43,7 @@ as root:
 ## Requirements
 
 - Host for testing framework: `python3`, `wrk`, `ab`, `nghttp2`, `h2spec`, 
-`curl`, `h2load`, `tls-perf`, `netstat`, `docker.io`, `nginx`, web content 
+`curl`, `h2load`, `tls-perf`, `netstat`, `lxc`, `nginx`, web content 
 directory accessible by nginx, nginx should not be running before the tests start.
 See Python libraries in `requirements.txt`
 - All hosts except previous one: `sftp-server`