Allow project credential entry to be an environment variable

When creating a manifest with project information to use with `wskdeploy sync`, it requires credentials to be specified in the `credential` entry. If I specify an environment variable, I get an error on deployment, whereas if I enter the credential in clear it works. In order to avoid credentials being leaked into files, this field should accept an environment variable that can be set externally.

## Steps to reproduce

1. Create a manifest that starts:

```yaml
project:
apiHost: your API host
credential: $OW_CREDENTIAL
```

2. Set the `OW_CREDENTIAL` environment variable to the correct value
3. `wskdeploy sync`: fails with the following error message:

```
Error: Deployment did not complete sucessfully. Run `wskdeploy undeploy` to remove partially deployed assets.
Error: servicedeployer.go [1627]: [ERROR_WHISK_CLIENT_ERROR]: Error code: 145: The connection failed, or timed out. (HTTP status code 401): HTTP Response Status: 401 Unauthorized HTTP Response Body: {"error":"The supplied authentication is invalid","code":"c9d3da79290088b0f68ffe7fe92d3a98"}
```

If the `credential` entry has the credential value in clear text, it works fine.

## Expected behaviour

It should be possible to set the value of `credential` to an environment variable that is set externally to avoid that value being stored in clear in a file that is under source control.

## Version

This was tried on `wskdeploy` `latest`, downloaded on 10th May.