Honor host level function filter

_From @mathewc on June 5, 2017 15:32_

As described in the host.json wiki https://github.com/Azure/azure-webjobs-sdk-script/wiki/host.json the host level "functions" filter allows the user to specify the set of functions that should be loaded/run. All other functions are ignored by the runtime.

It doesn't look like the portal honors this - we've had user reports that when they set this filter to a subset of their functions, they get portal errors.

We should also update the Kudu APIs as needed to honor this (e.g. sync triggers, etc.)

_Copied from original issue: Azure/azure-functions-ux#1428_

Start with the host.json wiki description of the host-level "functions" filter, then inspect how the runtime, portal, and Kudu APIs handle the filtered function set. Reproduce the reported portal errors and check related operations such as trigger synchronization. Done means filtered-out functions are ignored consistently by the host and relevant APIs.

Written by the indexing model from the issue text.