Load core faasd service definitions from compose

**What**
- Use the compose-go library to read the service definitions from an
  external compose file instead of building them in Go
- Add default compose file and copy during `faasd install`
- Add test for load and parse of compose file
- Make testing easier  by sorting the env keys
- Allow append to instantiate the slices so that we can more easily test
  for proper parsing (e.g. nil is still nil etc)
- Add the arch suffix to the compose file and set this as part of the
  env when we parse the compose file. This allows faasd to dynamically
  set the arch suffix used for the basic auth and the gateway images.

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

vendor/github.com/mattn/go-shellwords/go.mod

@@ -0,0 +1,3 @@
+module github.com/mattn/go-shellwords
+
+go 1.13