Add a common function to check for all environment variables needed to
test VSphere image. Call the function from all S3 implementations of
`checkEnv()` if testing the VSphere image type.
Move the installation of the VSphere client out of the
`api/common/aws.sh` `installClient()` function into a dedicated
`installClientVSphere()` function in `api/common/vsphere.sh`.
Rename the `installClient()` in `api/common/aws.sh` to
`installAWSClient()`. Introduce `installClient()` in both,
`api/aws.sh` and `api/aws.s3.sh`, which calls `installAWSClient()`.
Call `installClientVSphere()` conditionally when testing VMDK image type
in `api/aws.s3.sh` and `api/generic.s3.sh`.
