fix issue of the sp multiple calls for each partitions

We noticed that the services try to fetch app-dev-sp-username from keyvault for the first call for each partition. Especially it has an effect when there are a lot of partitions on the environment. Because of this we updated the TenantFactoryImpl not to retrieve the app-dev-sp-username on initPartition() call but use the initially fetched value from AzureOSDUConfig

Similar issue was found for the redis-hostname and redis-password. We used to do unnecessary KeyVault requests during checking whether the values presented into the KeyVault. Now it fixed as well.