use yarn workspaces to speed up the build

[akosyakov]

lerna bootstrap is dead slow:

che-theia-samples/package.json

Line 8 in 92f5d80

"prepare": "yarn bootstrap && lerna run prepare",

[benoitf]

I think there is some history behind : It was impossible to package plug-ins with yarn workspaces. (it has been fixed in plug-in packager) so lerna can go away now)