Set leader election deadline to 30s

[stefanprodan]

This PR allows fine tuning the leader election with cmd args:

--enable-leader-election=true
--leader-election-release-on-cancel=true
--leader-election-lease-duration=35s
--leader-election-renew-deadline=30s
--leader-election-retry-period=5s

To make the leader election more tolerant to transient Kubernetes API outages, the renewal deadline was increased from 10s to 30s and the retry interval was bumped to 5 seconds.

Test image:

docker.io/stefanprodan/source-controller:ls-config-a034c5d

Part of: fluxcd/pkg#94
Ref: #317