Bootstrap initializes the necessary vSphere objects before the tests are
run. A call to Bootstrap was missing in persistent_volumes-vsphere.go's
BeforeEach. This results in Panic while running e2e tests for 'vsphere'
provider with a stack trace like this:

  	/usr/local/go/src/runtime/panic.go:502 +0x229
  github.com/docker/kube-e2e-image/vendor/k8s.io/kubernetes/test/e2e/storage/vsphere.glob..func1.1()
  	/go/src/github.com/docker/kube-e2e-image/vendor/k8s.io/kubernetes/test/e2e/storage/vsphere/persistent_volumes-vsphere.go:77
+0xa21
  github.com/docker/kube-e2e-image/vendor/github.com/onsi/ginkgo/internal/leafnodes.(*runner).runSync(0xc4217c9b60,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
  	/go/src/github.com/docker/kube-e2e-image/e2e_test.go:88 +0x2c8
  testing.tRunner(0xc4206e01e0, 0x4212900)
  	/usr/local/go/src/testing/testing.go:777 +0xd0
  created by testing.(*T).Run
  	/usr/local/go/src/testing/testing.go:824 +0x2e0

This change fixes the Panic by calling Bootstrap.

Testing:
After this change, tests with FOCUS set to "PersistentVolumes:vsphere"
 dont Panic. They pass as expected.
Signed-off-by: Anusha Ragunathan <anusha.ragunathan@docker.com>