Enable tests against existing workload cluster

[jsturtevant]

What type of PR is this?
/kind feature

What this PR does / why we need it:
When developing new e2e test specs or working on the framework it is time consuming to wait for a cluster to be provisioned. This enables reusing existing workload cluster:

Assuming you already have a kind management cluster and ran e2e once which created a workload cluster called capz-e2e-uf9yll:

export LOCAL_ONLY=true 
export SKIP_CLEANUP=true 
export GINKGO_FOCUS=".*Windows.Enabled.*" 
export WINDOWS=true
export CLUSTERNAME=capz-e2e-uf9yll
export SKIP_CREATE_MGMT_CLUSTER=true
export KUBECONFIG=/home/jstur/projects/cluster-api-provider-azure/kubeconfig.e2e

./scripts/ci-e2e.sh

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

NONE

[jsturtevant]

/assign @chewong @mboersma @nader-ziada

[jsturtevant]

the windows failure was

Delete "https://capz-e2e-7xp181-620890e5.westus2.cloudapp.azure.com:6443/api/v1/namespaces/default/services/webqki40r-elb": http2: client connection lost

Haven't seen this error before. will keep an eye out for it
/test pull-cluster-api-provider-azure-e2e-windows

[CecileRobertMichon]

/lgtm
/approve

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[jsturtevant]

The failure here was #1328 which i've had some success fixing in #1351 (comment)

/test pull-cluster-api-provider-azure-e2e-windows

[jsturtevant]

/test pull-cluster-api-provider-azure-e2e-windows