fix: resource/project_ip_whitelist - modify ip whitelist entry valida…

[marinsalinas]

…tion and fix acctest

Description

Please include a summary of the fix/feature/change, including any relevant motivation and context.
Closes #252

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 requirments
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

--- PASS: TestAccResourceMongoDBAtlasProjectIPWhitelist_SettingMultiple (139.76s)
PASS
coverage: 4.7% of statements
ok      github.com/terraform-providers/terraform-provider-mongodbatlas/mongodbatlas     (cached)        coverage: 4.7% of statements

[PacoDw]

LGMT!

[coderGo93]

LGTM, only needs to be fixed for travis check

[themantissa]

lgtm - thank your for staying on this one