Multiple namespaces for secrets\configMaps don't work.

[Krasttt]

Multiple namespaces for secrets\configMaps don't work.

There is problem with secret\configMap slice, if we set two or more namespaces cert-exporter get cert just from last namespace from list. Another certs from other namespaces will be lost.

Changes:

• configMaps = c.Items to configMaps = append(configMaps, c.Items...)
• secrets = s.Items to secrets = append(secrets, s.Items...)
• Adds log about namespaces that will be scan if set parameter for it
• Adds log error for statement when cert-exporter get secret/configMap with labelSelector

[joe-elliott]

Nice catch!

Fixes #73