Fix reading Vault KVv2 secrets metadata

[findkim]

Fixes reading Vault KVv2 secrets metadata by not prepending /data/ path.

Test results before change

=== RUN   TestVaultReadQuery_Fetch_KVv2/read_metadata
    vault_read_test.go:407: 
        	Error Trace:	vault_read_test.go:407
        	Error:      	Received unexpected error:
        	            	no secret exists at read_fetch_v2/data/metadata/foo/bar
        	            	vault.read(read_fetch_v2/metadata/foo/bar)
        	            	github.com/hashicorp/consul-template/dependency.(*VaultReadQuery).Fetch
        	            		/Users/kngo/dev/hashicorp/consul-template/dependency/vault_read.go:80
        	            	github.com/hashicorp/consul-template/dependency.TestVaultReadQuery_Fetch_KVv2.func2
        	            		/Users/kngo/dev/hashicorp/consul-template/dependency/vault_read_test.go:406
        	            	testing.tRunner
        	            		/usr/local/go/src/testing/testing.go:1039
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1373
        	Test:       	TestVaultReadQuery_Fetch_KVv2/read_metadata

Resolves #1396

[eikenb]

While this looks good I did come up with a suggestion. While looking at the code it bugged me a bit that "metadata" was hardcoded in there but I realized it wasn't that that which bothered me but rather that it was that apiPrefix's value wasn't. The addPrefixToVKVPath function is called exactly once, in vault_read.go, and passed with apiPrefix set to "data".

My suggestion is then to remove the apiPrefix argument and hardcode "data" in there. Also maybe even move it into vault_read.go, the only place it is used.

What do you think?

[findkim]

@eikenb I'm glad you brought that up! I'm been fighting with preserving the existing code/logic. The apiPrefix parameter makes the function more obscure to handle and read. I can update this now

[eikenb]

Looks great and I like the new name.