Description of problem:
The "ipa vault-archive" command overwrites an existing value without any
warning.



Version-Release number of selected component (if applicable):
ipa-admintools-4.2.0-15.el7_2.15.x86_64


How reproducible:
Always.



Steps to Reproduce:
1. Add a value to a vault:
   # ipa vault-archive my-first-vault --data AbcD3fg8
   -----------------------------------------
   Archived data into vault "my-first-vault"
   -----------------------------------------

2. Verify that the value was stored in the vault:
   # ipa vault-retrieve my-first-vault
   ------------------------------------------
   Retrieved data from vault "my-first-vault"
   ------------------------------------------
     Data: AbcD3fg8

3. Re-run the "ipa vault-archive" with a different value for --data:
   # ipa vault-archive my-first-vault --data dmVyeSBzZWNyZXQ=
   -----------------------------------------
   Archived data into vault "my-first-vault"
   -----------------------------------------

4. Verify what's in the vault:
   # ipa vault-retrieve my-first-vault
   ------------------------------------------
   Retrieved data from vault "my-first-vault"
   ------------------------------------------
     Data: dmVyeSBzZWNyZXQ=



Actual results:
Existing data in the vault is overwritten without any notice.


Expected results:
If there is already data in the vault, the user should be notified or ask for
confirmation before overwriting.