INTMDB-252: Added two parameters for cluster advanced configuration

[coderGo93]

Description

• Added two parameters called default_read_concern and default_write_concern for cluster under advanced_configuration
• Added test TestAccResourceMongoDBAtlasCluster_basic_DefaultWriteRead_AdvancedConf
• Passed nil for create a project

Results of TestAccResourceMongoDBAtlasCluster_basic_DefaultWriteRead_AdvancedConf

 make testacc TESTARGS='-run=TestAccResourceMongoDBAtlasCluster_basic_DefaultWriteRead_AdvancedConf'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... | grep -v /integrationtesting) -v -parallel 20 -run=TestAccResourceMongoDBAtlasCluster_basic_DefaultWriteRead_AdvancedConf -timeout 300m -cover -ldflags="-s -w -X 'github.com/mongodb/terraform-provider-mongodbatlas/version.ProviderVersion=acc'"
?       github.com/mongodb/terraform-provider-mongodbatlas      [no test files]
=== RUN   TestAccResourceMongoDBAtlasCluster_basic_DefaultWriteRead_AdvancedConf
=== PAUSE TestAccResourceMongoDBAtlasCluster_basic_DefaultWriteRead_AdvancedConf
=== CONT  TestAccResourceMongoDBAtlasCluster_basic_DefaultWriteRead_AdvancedConf
--- PASS: TestAccResourceMongoDBAtlasCluster_basic_DefaultWriteRead_AdvancedConf (767.71s)
PASS
coverage: 8.2% of statements
ok      github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas 768.020s        coverage: 8.2% of statements
?       github.com/mongodb/terraform-provider-mongodbatlas/version      [no test files]

Link to any related issue(s):

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the Terraform contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

[andreaangiolillo]

LGTM