Enforce Credentials from each package in manifest file

When we added support for `packages` in manifest file in which it can have multiple `packages` with multiple `credentials` i.e. deploying each package to different openwhisk instance.

**Manifest file:**

```
packages:
helloworld-1:
namespace:
credential:
apiHost:
actions:
...
helloworld-2:
namespace:
credential:
apiHost:
actions:
...
```

This functionality is not currently supported which needs to be fixed. At this point,in this kind of use case `wskdeploy` ignores `credentials` from the manifest file if it has more than one packages.