New tool to bump current_versions.sh automatically

This commit contains a new utility that helps to automate a version bump
for sandbox.

You can run this command to get the vibe of what it does.

```
$ go run cmd/bump-version/main.go -help
```

In order to try it out you can run this command form sandbox root. By
default it won't overwrite anything. It will print to stdout a new
version of the current_versions.sh file where all the images are
calculate cloning the various repositories

```
$ go run cmd/bump-version/main.go
```

If you want to overwrite the current_versions file you can use the flag
`-overwrite`.

More will come but for now, that's the PoC. Ideally this can be hooked
to CI/CD and run periodically, opening a PR that can be evaluated and
merged.

Signed-off-by: Gianluca Arbezzano <gianarb92@gmail.com>

cmd/bump-version/envfile/envfile.go

@@ -0,0 +1,48 @@
+package envfile
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+
+	"github.com/joho/godotenv"
+)
+
+type EnvFile map[string]string
+
+func ReadEnvFile(f string) (EnvFile, error) {
+	myEnv, err := godotenv.Read(f)
+	if err != nil {
+		return nil, err
+	}
+	return myEnv, nil
+}
+
+// Copied and modified from https://github.com/joho/godotenv
+
+const doubleQuoteSpecialChars = "\\\n\r\"!$`"
+
+func Marshal(envMap map[string]string) (string, error) {
+	lines := make([]string, 0, len(envMap))
+	for k, v := range envMap {
+		lines = append(lines, fmt.Sprintf(`export %s="%s"`, k, doubleQuoteEscape(v)))
+	}
+	sort.Strings(lines)
+	return strings.Join(lines, "\n"), nil
+}
+
+func doubleQuoteEscape(line string) string {
+	for _, c := range doubleQuoteSpecialChars {
+		toReplace := "\\" + string(c)
+		if c == '\n' {
+			toReplace = `\n`
+		}
+		if c == '\r' {
+			toReplace = `\r`
+		}
+		line = strings.Replace(line, string(c), toReplace, -1)
+	}
+	return line
+}
+
+// End of the part copied from https://github.com/joho/godotenv