test/api: move govc installation to common/vsphere.sh

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`.

test/cases/api/aws.sh

@@ -27,6 +27,9 @@ function cleanup() {
   fi
 }
 
+function installClient() {
+  installAWSClient
+}
 
 function createReqFile() {
   AWS_SNAPSHOT_NAME=${TEST_ID}