Now that we are putting the sensitive jenkins and nodepool database
credentials in the new secure.conf file, let's remove the sensitive
information from nodepool.yaml.
Depends-On: Ifa4d500c52974b9fd3a0b7fd9c28cf8f52899ca9
Change-Id: Ib045c104daac15360e62f46e3e5683c7a387bc96