Refactor exec and buildformat to improve type safety and clearer imports

**What**
- Create and move exec commands to a new exec package, this will make it
easier to reuse in the future "install" command
- Move git related commands to the versioncontrol package to keep the
git related commands together and easy to find.
- Implement flags interfaces for the BuildFormat so that the flags
package can set and validate this value directly and then we can
directly pass this value around to improve type safety in the throughout
- Renames the `tag` variable to `tagFormat` to improve readability, the
value is not directly the tag value and this could be confusing before.

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

schema/image.go

@@ -1,9 +1,11 @@
 package schema
 
 import (
+	"fmt"
 	"strings"
 )
 
+// BuildFormat defines the docker image tag format that is used during the build process
 type BuildFormat int
 
 // DefaultFormat as defined in the YAML file or appending :latest
@@ -18,6 +20,48 @@ const BranchAndSHAFormat BuildFormat = 2
 // DescribeFormat uses the git-describe output as the docker tag
 const DescribeFormat BuildFormat = 3
 
+// Type implements pflag.Value
+func (i *BuildFormat) Type() string {
+	return "string"
+}
+
+// String implements Stringer
+func (i *BuildFormat) String() string {
+	if i == nil {
+		return "latest"
+	}
+
+	switch *i {
+	case DefaultFormat:
+		return "latest"
+	case SHAFormat:
+		return "sha"
+	case BranchAndSHAFormat:
+		return "branch"
+	case DescribeFormat:
+		return "describe"
+	default:
+		return "latest"
+	}
+}
+
+// Set implements pflag.Value
+func (l *BuildFormat) Set(value string) error {
+	switch strings.ToLower(value) {
+	case "", "default", "latest":
+		*l = DefaultFormat
+	case "sha":
+		*l = SHAFormat
+	case "branch":
+		*l = BranchAndSHAFormat
+	case "describe":
+		*l = DescribeFormat
+	default:
+		return fmt.Errorf("unknown image tag format: '%s'", value)
+	}
+	return nil
+}
+
 // BuildImageName builds a Docker image tag for build, push or deploy
 func BuildImageName(format BuildFormat, image string, version string, branch string) string {
 	imageVal := image