Only validate config for enabled signers

As we gain support for additional signers, that may or may not be enabled, it obviously doesn't work to validate the config for something that isn't configured. So validation must be configurable, preferably on the "signers.{name}.enabled" key. I suggest something like this:

```
func ValidateBySection(config *Config, cfgfile string) error {
var configsections = make(map[string]interface{}, 5)
validate := validator.New()

log.Printf(`
*** Config validation: Note that sections that do not have an 'enabled=true' key
*** are not validated, but may still be used.`)

configsections["signers.desec"] = config.Signers.DeSEC
configsections["signers.foobar"] = config.Signers.Foobar

for k, data := range configsections {
if viper.GetBool(fmt.Sprintf("%s.enabled", k)) {
log.Printf("Validating config for %s section", k)
if err := validate.Struct(data); err != nil {
log.Fatalf("Config %s, section %s: missing required attributes:\n%v\n", cfgfile, k, err)
}
} else {
log.Printf("Config %s, section %s: not enabled (requires %s.enabled=true), not validating its config.",
cfgfile, k, k)
}
}
e=return nil
}
```