Support shutdown of bootstrap server

[welteki]

Description

The call to bootstrap.Serve starts the provider server and blocks. In order to shutdown the server gracefully a parameter that accepts a context was added to support cancellation.

Motivation an context

This is required by the faas-netes provider to support graceful shutdown and should be useful for other provider implementation as well.

How has this been tested

This change was vendored into the faas-netes provider using the replace directive.

Verified the server shuts down when the context is done.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

All providers should be updated to call Serve with an additional context parameter.

[alexellis]

Approved