Add signing of certificates to cluster and downloading CA from cluster

As action for cluster, add certificate registration function and downloading function.

- sign certificate
  read csr file at browser and send magnum-ui REST api with cluster (id or name).
  use magnumclient.v1.certificates.create(csr='csr string', cluster='cluster id or name')
  magnum-ui REST api returns cert.pem as string to browser, and provide it to user as downloaded file by javascript.
- download CA
  use magnumclient.v1.certificates.get(cluster='cluster id or name')
  magnum-ui REST api returns ca.pem as string to browser, and provide it to user as downloaded file by javascript.
- help strings
  from magnumclient.v1.certificates_shell.py
- add actions to cluster table view and cluster detail view