[Feature] Load Stage-API-Config from subfolder

[cwiechmann]

User story
Actually, when giving Swagger-Promote a stage (e.g. -s prod), it's trying to load the Stage-API-Config based on the given API-Config in the same folder.
Primary config: myapi-config.json
Stage config: myapi-config.prod.json

But when using it as part of a pipeline, it might be desirable to have the following folder structure, that controls all Non-Dev-Stages:

|-- anotherapi
|   |-- pom.xml
|   |-- prod
|   |   |-- another-image-for-prod.png
|   |   |-- myapi-config.prod.json
|   |   |-- pom.xml
|   |   `-- prod-server-certificate.crt
|   `-- tst
|       |-- myapi-config.tst.json
|       |-- pom.xml
|       `-- test-certificate.crt
`-- myapi
    |-- pom.xml
    |-- prod
    |   |-- another-image-for-prod.png
    |   |-- myapi-config.prod.json
    |   |-- pom.xml
    |   `-- server-certificate-prod.crt
    `-- tst
        |-- myapi-config.tst.json
        |-- pom.xml
        `-- test-server-certificate.crt

So, the feature request is to support to load Stage-Config-Files also from a sub-folder if not found close to the main-config file.

[cwiechmann]

Is part of release 1.6.6