Use different Ubuntu pools for Ubuntu tests to avoid disk space issues.

- Updated the `default-build.yml` to have a new parameter specific to Ubuntu listed `useHostedUbuntu`. If we feel the need to expand the parameters usage outside of the Ubuntu configuration we can always rename to `useHosted`. Also didn't want to touch the `isTestingJob` because I wasn't sure of hte implications.
- Updated the Ubuntu test job to turn off hosted pools.

Fixes https://github.com/dotnet/aspnetcore-internal/issues/3574

Author: N. Taylor Mullen

.azure/pipelines/ci.yml

@@ -527,6 +527,7 @@ stages:
       jobDisplayName: "Test: Windows Server 2016 x64"
       agentOs: Windows
       isTestingJob: true
+      useHostedUbuntu: false
       buildArgs: -all -pack -test -BuildNative "/p:SkipHelixReadyTests=true /p:SkipIISNewHandlerTests=true /p:SkipIISTests=true /p:SkipIISExpressTests=true /p:SkipIISNewShimTests=true /p:RunTemplateTests=false" $(_InternalRuntimeDownloadArgs)
       beforeBuild:
       - powershell: "& ./src/Servers/IIS/tools/UpdateIISExpressCertificate.ps1; & ./src/Servers/IIS/tools/update_schema.ps1"

.azure/pipelines/jobs/default-build.yml

@@ -83,6 +83,7 @@ jobs:
     enableTelemetry: true
     helixRepo: dotnet/aspnetcore
     helixType: build.product/
+    useHostedUbuntu: true
     workspace:
       clean: all
     # Map friendly OS names to the right queue
@@ -91,7 +92,15 @@ jobs:
       ${{ if eq(parameters.agentOs, 'macOS') }}:
         vmImage: macOS-10.14
       ${{ if eq(parameters.agentOs, 'Linux') }}:
-        vmImage: ubuntu-16.04
+        ${{ if eq(parameters.useHostedUbuntu, true) }}:
+          vmImage: ubuntu-16.04
+        ${{ if eq(parameters.useHostedUbuntu, false) }}:
+          ${{ if eq(variables['System.TeamProject'], 'public') }}:
+            name: NetCorePublic-Pool
+            queue: buildpool.ubuntu.1604.amd64.open
+          ${{ if eq(variables['System.TeamProject'], 'internal') }}:
+            name: NetCoreInternal-Pool
+            queue: buildpool.ubuntu.1604.amd64
       ${{ if eq(parameters.agentOs, 'Windows') }}:
         ${{ if eq(variables['System.TeamProject'], 'public') }}:
           name: NetCorePublic-Pool