EZP-32214: Refactored config/packages/overrides/* to load from Extension

[webhdx]

Question	Answer
JIRA issue	EZP-32214
Improvement	yes
New feature	no
Target version	master
BC breaks	no
Tests pass	yes
Doc needed	no

Second attempt at refactoring config/packages/overrides/*.php files. Previously it wasn't fully working. It has to be a part of Extension::load in order for the data to be ready for compiler passes.

[adamwojs]

As we discussed on Slack: would be good to move this code into dedicated platform.sh related package. But for now it's OK.

[alongosz]

Some refactoring in the future of this code would be much appreciated. I'll try to find some time.

[mnocon]

First, a small thing:

1. there's no handling of fastly purge_server (Fastly purge_server is not set:
   https://github.com/ezsystems/ezplatform-ee/blob/master/config/packages/overrides/generic.php#L42-L44) which was a nice addition (and could break for people doing an upgrade).

2. SESSION_SAVE_PATH and SESSION_HANDLER_ID seem to have no effect

For:

export SESSION_SAVE_PATH=tcp://redis-session:6379?weight=1
export SESSION_HANDLER_ID=ezplatform.core.session.handler.native_redis

php bin/console debug:config FrameworkBundle still shows current values:

    session:
        handler_id: session.handler.native_file
        save_path: /Users/mareknocon/Desktop/Sites/v3_2/var/sessions/dev

but it should show (current behaviour):

session:
    handler_id: ezplatform.core.session.handler.native_redis
    save_path: 'tcp://redis-session:6379?weight=1

I've also tested Commerce on platform.sh:
3) Purge type for Varnish is not configured automatically.

After Platform.sh deployment purge_type still seems to be "local".
If I understand correctly this part of code executes only if the HTTPCACHE_PURGE_TYPE variable is not set: https://github.com/ezsystems/ezplatform-core/pull/27/files#diff-f72b922417794d0e5c2291bf6eddcfdf0e1e32a18d00f5ff3e8b46c514baa100R280
and now we give it a default value.

4. Solr is not configured when deploying on Platform.sh. It is set if I add SEARCH_ENGINE=solr variable to .platform.app.yaml. but without it it does not happen.

I will send you my Platform.sh project on Slack.

[mnocon]

QA approved.

Please remember to remove TMP changes in ezsystems/ezplatform#632 before merging.