We now have two write interceptors, with one moved into
faas-provider. This commit makes the gateway use the new
external package and deletes its own.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alex@openfaas.com>

This patch enables the use-case for multiple namepsaces by
allowing a dot to be used in the function name.

dep has been run to update OpenFaaS projects and also to
prune unused files.

Tested by doing a build.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>