Plugin broker should respect cluster CA certificates

[mmorhun]

Describe the bug

Che Workspaces failed to start due to plugin broker failure to reach a resource.
This happens when a resource is secured by different than Che server TLS certificate. This is the case when a proxy with SSL Bump is used.

Plugin broker logs:

2020/08/03 12:59:17 Broker configuration
2020/08/03 12:59:17   Push endpoint: wss://che-che.apps.user.devcluster.openshift.com/api/websocket
2020/08/03 12:59:17   Auth enabled: true
2020/08/03 12:59:17   Runtime ID:
2020/08/03 12:59:17     Workspace: workspace80pduq4q87wwoc9i
2020/08/03 12:59:17     Environment: default
2020/08/03 12:59:17     OwnerId: da658300-64b7-4700-96d9-8affd00484ff
2020/08/03 12:59:17   Self signed certificate /tmp/che/secret/ca.crt
2020/08/03 12:59:18 Couldn't connect to endpoint 'wss://che-che.apps.user.devcluster.openshift.com/api/websocket', due to error 'x509: certificate signed by unknown authority'

Workspace start logs:

Successfully assigned myusername-che/workspace80pduq4q87wwoc9i.che-plugin-broker to ip-10-0-159-97.us-east-2.compute.internal
Successfully assigned myusername-che/workspace80pduq4q87wwoc9i.che-plugin-broker to ip-10-0-159-97.us-east-2.compute.internal
Successfully assigned myusername-che/workspace80pduq4q87wwoc9i.che-plugin-broker to ip-10-0-159-97.us-east-2.compute.internal
Successfully assigned myusername-che/workspace80pduq4q87wwoc9i.che-plugin-broker to ip-10-0-159-97.us-east-2.compute.internal
Pulling image "quay.io/eclipse/che-plugin-metadata-broker:v3.2.0"
Successfully pulled image "quay.io/eclipse/che-plugin-metadata-broker:v3.2.0"
Created container che-plugin-metadata-broker-v3-2-0
Started container che-plugin-metadata-broker-v3-2-0
Error: Failed to run the workspace: "Plugins installation process timed out"

Che version

nightly

Steps to reproduce

Before trying to reproduce special environment should be setup.

1. Start a cluster with Openshift 4.
2. Setup a public proxy with SSL Bump (authorization is not mandatory).
3. Make Openshift cluster use the proxy.
4. Deploy Eclispe Che
5. Try to start worksace, see the error.

Some pitfalls:

1. To be able to deploy Che sometimes it is needed to add noProxy: api.user.devcluster.openshift.com,localhost into cluster proxy config.
2. If proxy do not support websockets (or not configured to proxy websocket connection) add into Eclispe Che CR:

spec:
  server:
    nonProxyHosts: devcluster.openshift.com

Expected behavior

Plugin broker trusts cluster CA certificates.
(Should be mounted into /public-certs, see #17407).

Runtime

Openshift
Client Version: 4.3.1
Server Version: 4.4.6

Installation method

chectl
chectl server:start --platform=openshift --os-oauth

Environment

Tested on Amazon, but should be reproducible on others if all requirements are met.

Additional context

[mmorhun]

Done