Provide a way to specify externals in the server build

[vikerman]

Currently we set --bundleDependencies all in the default schematics.

This is causing issues in certain libraries that don't bundle well. See #1207, #1195

The workaround is to go back to --bundleDependencies to 'none' and ship all of node_modules.

But it might be worthwhile to explore whether we can expose some mechanism to add certain imports as 'external' in the server builder config so that only those node_modules can be selectively shipped (Though that is up to the user to do that correctly)

[alan-agius4]

Address via angular/angular-cli#16359

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