Allow hiding of specific startup warnings via config params

Some production deployments are forced to run ext4 filesystems (for example) due to legal/licensing reasons. This causes a startup warning on every node. When using monitoring tools (such as Ops Manager), this can create false positives and admins may be tempted to disable startup warning checks completely.

I'm requesting a new configuration option to disable specific startup warnings, so that remaining warnings are true positives and don't spam Ops Manager alerts etc.

I envisage a new configuration file option like systemLog.hideWarnings which takes an array of strings like {{"dbPath-ext4"} that disables just the ext4 dbPath volume warning, but leaves all others in place.

This will require naming every warning and adding logic in startup_warnings_mongod.cpp and startup_warnings_common.cpp.