Change DWD prober to check Node Leases

[rishabh-11]

What this PR does / why we need it:
This PR removes the old logic of DWD prober doing two probes, internal and external. The internal probe still happens and is renamed to APIServerProbe, and a NodeLease probe is introduced, which replaces the external probe. It takes a look at the number of expired Node Leases and performs a scale-down if the threshold is crossed. Refer #85 to see why this is required.

Which issue(s) this PR fixes:
Fixes #85

Special notes for your reviewer:

Release note:

DWD no longer depends on checking the reachability of Kube-API server via external load-balancer as kube-proxy path optimizations makes that impossible on infrastructures which have a static IP for the LB. Instead it now depends on node leases that are periodically renewed by each node running in the shoot cluster to determine its scaling action (scale-down/up).

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[rishabh-11]

The testing of this PR was done on AWS and GCP. Attaching files for AWS and GCP logs, DWD config map and rootboy.txt (used for shutting down kubelet) :-

AWS.md
GCP.md
dwd-cm.txt
rootboy.txt

[gardener-prow]

LGTM label has been added.

Git tree hash: d8bf7eae8c15af6cae090bc7da92d8719f471b8e

[gardener-prow]

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