Implements a user-experience changes:

faas-cli new - when the templates folder is not present the
language will be looked up in the template store using the
default URL or an overriden one.

faas-cli publish - when no template folder is available then
a pull will be carried out if a configuration section is
present with a template given within in

Filtering - whilst there is no way to avoid pulling all the
templates wihtin a repository, only the requested/needed ones
are expanded by commands like faas-cli new.

Tested mainly with existing-unit tests, faas-cli new / publish
with and without a templates folder.

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

The tests relied on ruby being a long term template in the
templates repository, however we are moving to a model
with a repository per template, and this template has gone
to EOL.

The Dockerfile template makes sense as a replacement and is
still supported.

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

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

Requested by @welteki, if stack.yml is present then it is
taken to be the default with a --yaml/-f argument being needed.

This change also looks for stack.yaml as a valid alternative
name.

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

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

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

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

err could be nil, so it is checked before checking for the
type of error. This may not be needed, but is a better
default to establish since code is often copied by new
contributors when adding features.

Addresses comment by @rgee0 for handling error from stat.

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

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

Updates faas-cli new to create a stack.yaml file instead of
a YAML file named after the function. This is to help users
discover that stack.yaml does not require a --yaml or -f
flag which can become cumbersome.

When a stack.yaml or the file specified via -f already exists
the new command will now prompt users to use --append

Convert to SHA256 signatures for the invoke command

This is a convenience option for invoking functions along with
a signature, if a function accepts one. They should all have been
moved to sha256 now if they were even using it

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

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