pkg/storage/azure: increase storage account key cache expiration

make it 20 minutes instead of 5.

Azure has an api limit for storage account list (which apparently
listing keys uses behind the scenes) of 100 calls / 5 minutes. if a
project has a lot of clusters, this limit might be hit.

pkg/storage/azure/cached_key.go

@@ -10,6 +10,9 @@ import (
 	"github.com/openshift/cluster-image-registry-operator/pkg/metrics"
 )
 
+// cacheExpiration is the cache expiration duration in minutes.
+const cacheExpiration time.Duration = 20 * time.Minute
+
 // primaryKey keeps account primary key in a cache.
 var primaryKey cachedKey
 
@@ -44,6 +47,6 @@ func (k *cachedKey) get(
 	k.resourceGroup = resourceGroup
 	k.account = account
 	k.value = *(*keysResponse.Keys)[0].Value
-	k.expire = time.Now().Add(5 * time.Minute)
+	k.expire = time.Now().Add(cacheExpiration)
 	return k.value, nil
 }