Impossible to change OpenApi document URL via application.properties

[belyaev-andrey]

Describe the bug
If you add a property quarkus.smallrye-openapi.path to change an OpenApi doc location (I don't like /openapi personally) it fails if you specify something like

quarkus.smallrye-openapi.path=/api/openapi.yaml

Error is:

java.io.FileNotFoundException: /api/quarkus-generated-openapi-doc.YAML
	at io.undertow.servlet.handlers.DefaultServlet.doGet(DefaultServlet.java:173)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:686

Expected behavior
OpenApi document is downloaded

Actual behavior
Error 500

To Reproduce
Steps to reproduce the behavior:

1. clone https://github.com/quarkusio/quarkus-quickstarts
2. cd to quarkus-quickstarts/using-openapi-swaggerui folder
3. open src/main/resources/application.properties
4. add quarkus.smallrye-openapi.path=/api/openapi line to this file
5. run the application by executing mvn compile quarkus:dev in the quarkus-quickstarts/using-openapi-swaggerui folder
6. Open http://localhost:8080/swagger-ui/ URL
7. Open http://localhost:8080/api/openapi

Observe the error

Configuration

# Configuration file
# key = value
quarkus.smallrye-openapi.path=/api/openapi

Environment (please complete the following information):

• Output of java -version:

java version "10.0.2" 2018-07-17
Java(TM) SE Runtime Environment 18.3 (build 10.0.2+13
Java HotSpot(TM) 64-Bit Server VM 18.3 (build 10.0.2+13, mixed mode)       

• Quarkus version or git rev: 0.16.1

Additional context
I think that the problem is that you don't take into account this configuration property while generating OpenApi file and creating the following constants:

    public static final String GENERATED_DOC_BASE = "quarkus-generated-openapi-doc.";
    public static final String BASE_NAME = "META-INF/resources/" + GENERATED_DOC_BASE;

You probably need to add config path property to the generated doc file name, therefore it should look like:

getConfigValue(quarkus.smallrye-openapi.path)+"quarkus-generated-openapi-doc."