Oracle aims to reach the top of the cloud, but its ascent won't be easy. The database leader's cloud push -- crystallized at its five-day, annual OpenWorld trade show that ended Thursday -- will require challenging disparate rivals ranging from Salesforce.com (CRM) to Amazon.com (AMZN). It might take years for Oracle's efforts to bring serious rain, some analysts say. Oracle's (ORCL) efforts to sell database software, applications, computing power