update build definition default agent pool name

[xuzhang3]

All Submissions:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My code follows the code style of this project.
• I ran lint checks locally prior to submission.
• Have you checked to ensure there aren't other open PRs for the same update/change?

What about the current behavior has changed?

Rename build definition default agent pool name from Ubuntu 16.04 to Azure Pipelines
Issue Number: #455

AccTests:

=== RUN   TestAccBuildDefinitionPermissions_SetPermissions
=== PAUSE TestAccBuildDefinitionPermissions_SetPermissions
=== RUN   TestAccBuildDefinitionPermissions_UpdatePermissions
=== PAUSE TestAccBuildDefinitionPermissions_UpdatePermissions
=== RUN   TestAccBuildDefinition_Create_Update_Import
=== PAUSE TestAccBuildDefinition_Create_Update_Import
=== RUN   TestAccBuildDefinitionBitbucket_Create
=== PAUSE TestAccBuildDefinitionBitbucket_Create
=== RUN   TestAccBuildDefinition_WithVariables_CreateAndUpdate
=== PAUSE TestAccBuildDefinition_WithVariables_CreateAndUpdate
=== RUN   TestAccBuildDefinition_Schedules
=== PAUSE TestAccBuildDefinition_Schedules
=== CONT  TestAccBuildDefinitionPermissions_SetPermissions
=== CONT  TestAccBuildDefinitionBitbucket_Create
=== CONT  TestAccBuildDefinition_Create_Update_Import
=== CONT  TestAccBuildDefinition_Schedules
=== CONT  TestAccBuildDefinitionPermissions_UpdatePermissions
=== CONT  TestAccBuildDefinition_WithVariables_CreateAndUpdate
--- PASS: TestAccBuildDefinition_WithVariables_CreateAndUpdate (67.76s)
--- PASS: TestAccBuildDefinitionPermissions_SetPermissions (88.36s)
--- PASS: TestAccBuildDefinition_Schedules (147.52s)
--- PASS: TestAccBuildDefinitionBitbucket_Create (185.53s)
=== CONT  TestAccBuildDefinitionPermissions_UpdatePermissions
--- PASS: TestAccBuildDefinitionPermissions_UpdatePermissions (208.84s)
--- PASS: TestAccBuildDefinition_Create_Update_Import (263.65s)
PASS
PASS    github.com/microsoft/terraform-provider-azuredevops/azuredevops/internal/acceptancetests        264.322s

Does this introduce a change to go.mod, go.sum or vendor/?

• Yes
• No

Does this introduce a breaking change?

• Yes
• No

Any relevant logs, error output, etc?

(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Other information