Add a registry package with registry v1/v2 code

This extract what was in registry_test.go and
registry_mock_test.go. This also move `RegistryHosting`
requirement to `registry.Hosting`

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

integration-cli/registry/requirement.go

@@ -0,0 +1,12 @@
+package registry
+
+import "os/exec"
+
+// Hosting returns wether the host can host a registry (v2) or not
+func Hosting() bool {
+	// for now registry binary is built only if we're running inside
+	// container through `make test`. Figure that out by testing if
+	// registry binary is in PATH.
+	_, err := exec.LookPath(v2binary)
+	return err == nil
+}