Add support for Local provider for e2e tests

[shreyas-s-rao]

How to categorize this PR?

/area testing quality
/kind test

What this PR does / why we need it:
Add support for Local provider for e2e tests, along with other minor fixes/cleanups.

Which issue(s) this PR fixes:
Fixes #667

Special notes for your reviewer:

1. Local provider backup buckets are created by mounting /etc hostPath onto the infra job in order to create ./gardener/local-backupbuckets/<container-name> directory and to delete it once the tests are completed. Skaffold + kustomize is used for this, similar to infra jobs of other providers like aws, azure and gcp.
2. Purging of snapshots at the beginning of each test spec was not working (v1 snapshots were being purged instead of v2 snapshots), due to which some tests were flaky, especially creation of 3-node etcd cluster, since backups possibly already existing in the bucket from a previous test spec. This has now been fixed, as well as enhanced for Local provider via a purge-local-snapstore job that runs before each test spec when using Local provider.
3. Occurrences of etcd-main in many tests have been replaced by etcd-test.
4. Repeated import of k8s.io/api/core/v1 package as corev1 and v1 in test/e2e/utils.go has been rectified.
5. PHONY for deploy-via-kustomize in Makefile has been fixed.
6. Update doc to include Local provider for e2e tests; minor edits in docs too.

This PR can be tested by running:

make kind-up
make PROVIDERS="local" TEST_ID="e2e-test" STEPS="setup,deploy,test,undeploy,cleanup" test-e2e
make USE_ETCD_DRUID_FEATURE_GATES=true PROVIDERS="local" TEST_ID="e2e-test-2" STEPS="setup,deploy,test,undeploy,cleanup" test-e2e
make kind-down

Release note:

Add support for `Local` provider for e2e tests.

[seshachalam-yv]

Overall looks good to me
Thanks @shreyas-s-rao for adding local provider support to E2E test
/lgtm

[aaronfern]

Thanks for the PR @shreyas-s-rao!
Looks good, just one question, otherwise /lgtm

[aaronfern]

/lgtm