Allow setting profile collection's size via a configuration option

The default size of a profile collection is tiny in MongoDB (related issue: https://jira.mongodb.org/browse/SERVER-4787), which forces most of us to increase it on our production systems.

In order to increase the profile collection size, we need to perform the awkward dance described in https://docs.mongodb.com/manual/tutorial/manage-the-database-profiler/#change-size-of-system-profile-collection-on-the-primary. It would be much easier if we could simply set its size as a config option, similar to replication.oplogSizeMB.