Add keyring_vault support to PXB framework --test_mode

[ShahriyarR]

1. xtrabackup will be able to talk to Vault server directly,
  keyring_vault plugin will be ported to xtrabackup, all the same
  options will be supported.
2. new option will be added called `--transition-key'. This key will be
  used to reencrypt keys stored in the InnoDB tablespace headers.
3. xtrabackup will perform following additional steps as it goes through
  encrypted tablespaces:
  - fetch the master key used to encrypt this specific tablespace and
    store it encrypted with transition key into a special file (to be
    named).
  - reencrypt the tablespace key with transition key.
4. xtrabackup will require user to specify `--transition-key' for
  apply-log/prepare/copy-back operation.
5. for `--copy-back' xtrabackup will reencrypt tablespaces so that they
  use the same keys as before and can be used with the same vault
  server.
6. optionally, if user wants to switch to the new key server or
  different keyring plugin, there will be option
  `--generate-new-master-key'. When this option is specified together
  with `--copy-back', xtrabackup will generate new master key, store it
  into keyring and reencrypt tablespaces with this new key.

[ShahriyarR]

The first thing to be added can be starting PS with keyring_vault plugin.

[ShahriyarR]

Implemented and merged.
Will be closed with release_v1.5.2