[Improve che-plugin.yaml] Use of the property mountProjectSources (or even the simpler mountSources) for a tool/plugin

Description

This issue is created as result of the following discussion https://github.com/redhat-developer/devfile/pull/2#discussion_r242876449

Now if plugin developer wants to get projects sources mounted to his plugin then he has to define volume with name projects and path /projects.

version: 1.0.0
type: Che Editor
name: theia-ide
id: org.eclipse.che.editor.theia
endpoints:
       ...
containers:
 - name: theia-ide
   image: eclipse/che-theia:0.3.18-nightly
   env:
       ...
   volumes:
       - mountPath: "/projects"
         name: projects
       - mountPath: "/plugins"
         name: plugins
   ports:
       ...

The idea of this issue is simplify defining of mounting projects sources to the following format:

version: 1.0.0
type: Che Editor
name: theia-ide
id: org.eclipse.che.editor.theia
endpoints:
       ...
containers:
 - name: theia-ide
   image: eclipse/che-theia:0.3.18-nightly
   env:
       ...
   volumes:
       - mountPath: "/plugins"
         name: plugins
   mountSources: true // false
   ports:
       ...

should define project volumes The idea of this issue is avoid defining projects volume by user/plugin developer.

The scope of this issue includes:

• Made Che Server respecting this flag and provision projects volume automatically with /projects path specified. Note that the solution should be back forward compatible and plugins that are configured in old way (with projects volume specified) should work. It may be implemented by taking mountSources into account only when projects volume is missing;
• Update field name in dockerimage tool when final name decision about mountSources/mountProjectSources will be made. See https://github.com/redhat-developer/devfile/blob/master/Devfile.yaml;

[metlos]

as per https://www.eclipse.org/lists/che-dev/msg03002.html I will go ahead and create new issues for the more fine-grained tasks towards implementing the agreed solution.

[metlos]

Should the attribute be named mountSources?