Add support for Vault KV secrets engine v2 (#86)

* Fix vault couldn't get a value

* Add support for Vault KV secrets engine v2

* Allow simpler paths like the Vault CLI does

* Fix Vault tests fail when run twice

Due to data versioning the deferred deletion doesn't completely remove
the secret - only the data.

* Improve path change

Co-authored-by: Jean-Philippe Moal <[email protected]>

Co-authored-by: mopemope <[email protected]>
Co-authored-by: Jean-Philippe Moal <[email protected]>

Tagging v0.9.2

update vault module version (#83)

By requiring the old version of vault, we were breaking dependency resolution because that version has an api package which conflicts with the later api module that has the same import path.

Also fix the error message checking so that it copes with the latest Go tip.

backend/env: treat empty environment variables as not found (#78)

It's very unusual to distinguish between an environment variable
that's unset and one that's empty. When a value is required, looking
for a non-empty environment variable feels more correct.

As a recent example, the kafka-go config contained a "required" entry
for `KAFKA_ADDRS` which was satisfied by an empty environment
variable.

Merge pull request #72 from heetch/rog-002-consul-v1.1.0

use consul v1.1.0

Merge pull request #59 from heetch/add-optional-parameter

Add optional parameter for file backend

Merge pull request #62 from heetch/rog-001-cleanup

code cleanups; more tests

Merge pull request #31 from heetch/fix-bug

Check if the flag has been set before to set the value.

Update README with slice support information

Add latest etcd, vault and consul usage