Extracting forceJumbo from docs in config.migrations in MigrationType::fromBSON can throw

A new field 'forceJumbo' was added to docs in config.migraitons in v4.4. In MigrationType::fromBSON, we attempt to extract this field from a BSONObj which will throw NoSuchKey if the field does not exist. We should prevent the server from throwing if the field does not exist and instead use the default value 'doNotForceJumbo'.