Volumes from DevWorkspace and DevWorkspaceTemplates should be merged

[sleshchenko]

The context:

1. In Che flow, user's devfile.yaml describe only their application related component, and everything related to tooling and IDE come from plugins;
2. DevWorkspace Operator powers that flow with DevWorkspaces(aka DW) (for devfile) and DevWorkspaceTemplates(aka DWT)(for plugins), and then use flattening devfile logic from devfile spec;
3. Flattening devfile logic requires for each devfile item(DW, DWT) (unless it's parent logic which we don't consider) provision objects(component, commands, projects) with unique names.

So, let's consider two cases in details:

1. [Upstream] User has java-maven application that has devfile with volume for maven repository, and java plugin that then just mounts that repo in its container;
2. [Downstream] Java plugin is designed in the common way to work for both of maven and gradle, so it mounts maven and gradle repos volumes, which requires from users to declare both volumes in their devfiles, but it does not make much sense for application to use both build tools at the same time.

So, it's proposed to allow plugin define their volumes and to make it working add DevWorkspace specific logic for processing DevWorkspaceTemplates volumes, which will work in the following way:

1. Process each DevWorkspaceTemplate volumes:
   1.1. Check if it's in DevWorkspace(user's devfile):
   • it's there - use one from DevWorkspace;
   • it's not there - use one from DevWorkspace Template;

Downstream issue https://issues.redhat.com/browse/CRW-2253