New fake Vault module for use with Isopod dry run

[nitishkrishna]

During an Isopod Dry Run, we don't want the Vault Client to actually read or write values to vault. Instead we would prefer if:

• Dry-Run validates that a path being read exists. Same for vault.exist
• Dry-Run doesn't write to a path during a dry-run (and instead just returns the data it is "supposed" to write).

Integration tests added to validate this behavior.