Add LeaseLock for leader election

[chala2001]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Leader election can currently only use a ConfigMap. client-go moved to the coordination.k8s.io Lease resource and removed ConfigMap support, so this adds a LeaseLock next to ConfigMapLock. The example now uses it, with the ConfigMap line kept commented out.

Which issue(s) this PR fixes:

Fixes #1877

Special notes for your reviewer:

This picks up @rvlane's #2314, which you reviewed with "LGTM overall. Could you add a test?" and which the triage robot later closed as rotten. The original work is credited with a Co-authored-by trailer. The tests you asked for are in leaselock_test.py.

Four things changed from that version while rebasing onto current master:

• Logger. The original called logging.basicConfig(level=logging.INFO) at import, which reconfigures the root logger of whatever application imports the library. This package moved to logger = logging.getLogger("leaderelection") in the meantime, so it uses that now.
• Time formats. str(datetime) leaves the microseconds out when they are exactly zero, and the original only accepted the %f spellings. Those times fell through and returned None, so the lease was written with a null acquireTime. Covered by test_record_without_microseconds_survives_a_write_and_read.
• lease_reference is kept after create, so a following update has something to work from. update also returns False rather than raising AttributeError when it is called before a get, which matches client-go's lease not initialized check.
• UTC. leaderelection.py builds its times with datetime.fromtimestamp(), which is naive local time. The original appended Z to it, which labels local wall clock as UTC and writes a timestamp that is wrong by the host's offset. Since the point of a Lease is that other clients read it, the times are now converted to the real UTC instant and converted back on read, so the election record still round trips unchanged. test_times_are_written_as_the_real_utc_instant pins that under Asia/Kolkata.

One thing I left out, happy to follow up if you would like it: leaseTransitions is neither read nor written, because LeaderElectionRecord has no field for it. client-go writes it on every update. Adding it would touch the shared record class and ConfigMapLock as well, so it seemed better as a separate change.

Verified: 17 tests pass under kubernetes/base/leaderelection (5 existing, 12 new), and reverting each of the four fixes above makes the matching test fail. pycodestyle --max-line-length=100 is clean on both new files. I have not run the functional or e2e suites.

Add LeaseLock for leader election, using the coordination.k8s.io Lease resource.

[chala2001]

@yliaog this one is ready when you get a chance. @Invictus17 reviewed it and approved, and I pushed the change he asked for in 6073f7f.

He isn't in OWNERS though, so Prow still shows it as not approved ,, it needs /lgtm and /approve from you or @fabianvf before it can go in..

[chala2001]

Fixed all three in 9b87f18 - both copyright years and the spelling. Thanks for the review.

[yliaog]

thanks for the PR

/lgtm
/approve

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chala2001, Invictus17, yliaog

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