Don't escape \n in secret dump

[vukor]

Special symbols like \n are double escaped and secret value with newlines is not identical after restoration.

How to reproduce:

1. create secret

vault write /secret/ssh-key key='-----BEGIN OPENSSH PRIVATE KEY-----
secret
-----END OPENSSH PRIVATE KEY-----
'

2. get secret

$ vault kv get -format=json  /secret/ssh-key
{
  "renewable": false,
  "data": {
    "key": "-----BEGIN OPENSSH PRIVATE KEY-----\nsecret\n-----END OPENSSH PRIVATE KEY-----\n"
  },
  "warnings": null
}

3. dump secret by vault-backup

$ TOP_VAULT_PREFIX=/secret/ssh-key/ python vault-dump.py                                                                                                                                                                               
#
# vault-dump.py backup
# dump made by vukor
# TOP_VAULT_PREFIX env variable: /secret/ssh-key/
# STDIN encoding: utf-8
# STDOUT encoding: utf-8
#
# WARNING: not guaranteed to be consistent!
#
vault write /secret/ssh-key key='-----BEGIN OPENSSH PRIVATE KEY-----\nsecret\n-----END OPENSSH PRIVATE KEY-----\n'

4. restore secret

$ vault write /secret/ssh-key key='-----BEGIN OPENSSH PRIVATE KEY-----\nsecret\n-----END OPENSSH PRIVATE KEY-----\n'
Success! Data written to: secret/ssh-key

5. get secret

$ vault kv get -format=json  /secret/ssh-key
{
  "renewable": false,
  "data": {
    "key": "-----BEGIN OPENSSH PRIVATE KEY-----\\nsecret\\n-----END OPENSSH PRIVATE KEY-----\\n"
  },
  "warnings": null
}

As you see it prints \\n. So in my use case, I've got the file with an incorrect SSH private key format.