Lease name should be configurable

[james-callahan]

For various reasons to work around csi implementation shortcomings, we need to run multiple concurrent csi-attacher daemons (one per region).
However the lock name is not configurable. Code at https://github.com/kubernetes-csi/external-attacher/blob/3a057e780fb51b10c54ae292e06b2fa3d8540a43/cmd/csi-attacher/
main.go#L348

For now we accomplish this by having each instance keep its leases in a different namespaces via --leader-election-namespace, but it's annoying to have to create and maintain various namespaces containing only a lease + the necessary rolebinding

We'd love a new command line flag/env var e.g. --leader-election-lease-name with the current value as the default.

[jsafrane]

With leader election arguments moving to csi-lib-utils, it could be easy. PRs are welcome.