rename the KES client env. variables for the client key and cert

This commit renames the client env. variables used to specify
the client certificate and private key.

Before, the CLI client expected:
```
export KES_CLIENT_TLS_CERT_FILE=<cert>
export KES_CLIENT_TLS_KEY_FILE=<key>
```

Now, the CLI client expects:
```
export KES_CLIENT_CERT=<cert>
export KES_CLIENT_KEY=<key>
```

The reason for this change is that the previous env. variables
are quite verbose and look odd compared to the `KES_SERVER`
variable.

The new variables are more compact and don't require boilerplate
typing.

It has been considered to have a fallback - like if `KES_CLIENT_CERT`
is not present look for `KES_CLIENT_TLS_CERT_FILE`. However, it
seems worse to have a deprecated fallback that gets removed at some
point in time. A clean cut seems better here.

cmd/kes/main.go

@@ -104,13 +104,13 @@ func isFlagPresent(set *flag.FlagSet, name string) bool {
 }
 
 func newClient(insecureSkipVerify bool) (*kes.Client, error) {
-	certPath := os.Getenv("KES_CLIENT_TLS_CERT_FILE")
-	keyPath := os.Getenv("KES_CLIENT_TLS_KEY_FILE")
+	certPath := os.Getenv("KES_CLIENT_CERT")
+	keyPath := os.Getenv("KES_CLIENT_KEY")
 	if certPath == "" {
-		return nil, errors.New("No client TLS certificate: env KES_CLIENT_TLS_CERT_FILE is not set or empty")
+		return nil, errors.New("No client TLS certificate: env KES_CLIENT_CERT is not set or empty")
 	}
 	if keyPath == "" {
-		return nil, errors.New("No client TLS private key: env KES_CLIENT_TLS_KEY_FILE is not set or empty")
+		return nil, errors.New("No client TLS private key: env KES_CLIENT_KEY is not set or empty")
 	}
 	cert, err := tls.LoadX509KeyPair(certPath, keyPath)
 	if err != nil {