Configure pg_hba in the local postgresql configuration of Patroni. (#361)

Previously, the operator put pg_hba into the bootstrap/pg_hba key of
Patroni. That had 2 adverse effects:
 - pg_hba.conf was shadowed by Spilo default section in the local
   postgresql configuration
 - when updating pg_hba in the cluster manifest, the updated lines were
   not propagated to DCS, since the key was defined in the boostrap
   section of Patroni.

Include some minor refactoring, moving methods to unexported when
possible and commenting out usage of md5, so that gosec won't complain.

Per https://github.com/zalando-incubator/postgres-operator/issues/330

Review by @zerg-junior

pkg/util/util.go

@@ -1,7 +1,7 @@
 package util
 
 import (
-	"crypto/md5"
+	"crypto/md5" // #nosec we need it to for PostgreSQL md5 passwords
 	"encoding/hex"
 	"math/rand"
 	"regexp"
@@ -48,7 +48,7 @@ func PGUserPassword(user spec.PgUser) string {
 		// Avoid processing already encrypted or empty passwords
 		return user.Password
 	}
-	s := md5.Sum([]byte(user.Password + user.Name))
+	s := md5.Sum([]byte(user.Password + user.Name)) // #nosec, using md5 since PostgreSQL uses it for hashing passwords.
 	return md5prefix + hex.EncodeToString(s[:])
 }
 
@@ -120,6 +120,7 @@ func MapContains(haystack, needle map[string]string) bool {
 	return true
 }
 
+// Coalesce returns the first argument if it is not null, otherwise the second one.
 func Coalesce(val, defaultVal string) string {
 	if val == "" {
 		return defaultVal