Move the emqx_config:update,remove,reset APIs to emqx, to remove
the circular dependency between the modules emqx_config and
emqx_config_handler.
After this change the dependency among these modules will be:
```
emqx ---> emqx_config
| ^
| |
+ ---> emqx_conifg_handler
```
* refator(config_handler): handle and validate the updates to raw_configs
* fix(hocon): update hocon to 0.8.0
* refactor(config_handler): check and apply envs only in top-level handler
* refactor(config_handler): update config from top level to bottom level
* refactor(emqx_data_bridge): move configs to emqx.conf
* fix(emqx_schema): remove the extra config path
* fix(config_handler): load the emqx.conf when starting emqx_config_handler
* fix(data_bridge): API not working
* feat(config_handler): save updated configs to emqx_override.conf
* fix(config_handler): cannot find the emqx.conf and emqx_override.conf
* fix(emqx_config): cannot find the correct path for etc dir
* fix(test): load load emqx_schema foreign refereced apps
* refactor(emqx_plugin): do not generate configs before load plugins
All configs (including the configs for plugins) now should go into
the `emqx.conf`.
* fix(tests): update the test cases for plugins
* fix(tests): don't include schema from apps when testing
* fix(tests): use emqx-ct-helper branch hocon
