feat(igor/google): Support GCB cancellation from Spinnaker UI.

[himanhsugusain]

Description

Add support for stopping GCB (google cloud build) from Spinnaker.
The current GCB controller in Igor only supports Cancellation of builds global pool under the project, so does this change.

Testing

Local Spinnaker Instance testing with GCB.

# Test 
~/personal/orca master % ./gradlew :orca-igor:test

> Task :orca-igor:compileGroovy
warning: Implicitly compiled files were not subject to annotation processing.
  Use -implicit to specify a policy for implicit compilation.
Note: /Users/himanshu.gusain/personal/orca/orca-igor/src/main/java/com/netflix/spinnaker/orca/igor/tasks/StartGoogleCloudBuildTask.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 warning

BUILD SUCCESSFUL in 6s
25 actionable tasks: 3 executed, 22 up-to-date

[spinnakerbot]

The following commits need their title changed:

• 47ef382: Stop build for GCB

• c76197b: add test

Please format your commit title into the form:

<type>(<scope>): <subject>, e.g. fix(kubernetes): address NPE in status check

This allows us to easily generate changelogs & determine semantic version numbers when cutting releases. You can read more about commit conventions here.