Perform additional checks during repo bucket/user creation

[spiegela]

There are several situation, in which the repository bucket or user may not be created properly:

1. The repository-service or repository-plan is not set in the broker configuration.
2. The selected repository-service is encrypted, but the default namespace is not.
3. There is a name conflict in the repository-user

If the bucket is able to be created, without the user, it can require manual intervention to put the ECS components into a rational state before the broker can be started. We should perform more extensive checks during startup, to make sure that these issues don't arise.

[spiegela]

The desire behavior is that the broker should crash any time the required repository resources can't be created. The broker also needs to validate that the settings are all create, as customers have run into partially created environments when messing around with settings. Required resources include:

• Repository bucket
• Repository user
• Full control ACL for repository user on repository bucket
• Configured base URL or object-endpoint
• Replication group
• Namespace
• Management user and password