Support starting a workspace from directories without a .gitpod.yml

[csweichel]

Is your feature request related to a problem? Please describe.
Today the workdir needs to have a .gitpod.yml for run-gp to work.

Describe the solution you'd like
Calling run-gp on a directory without a .gitpod.yml could search up the directory tree for such a file. If it doesn't find one, we'd start with an "empty" default config.