feature request: provide a way to override stanzas from sub-modules

- Skaffold currently doesn't provide any ways to override fields from sub-modules, it would be great if skaffold could provide such feature, there are some use cases:
- users may want to override build environment from a submodule from local to google cloud build to leverage cloud power
- defaultNamespace in deployer stanza, a user can have multiple skaffold submodules and he wants to deploy images from different namespace, he can override those Default namespaces from the root modules
- A user doesn't have direct control over a module, he want some stuff from a remote module, but the module may be too heavy, the user can override/remove the stanza he doesn't need.
- may be also change the tagstrategy, and all kinds of hooks and more..