Fix return types

We're now stricter on naming the return types.  Replace string with str.

lib/ansible/modules/cloud/vmware/vmware_dvswitch_uplink_pg.py

@@ -132,7 +132,7 @@ RETURN = """
 result:
     description: information about performed operation
     returned: always
-    type: string
+    type: str
     sample: {
         "adv_block_ports": true,
         "adv_netflow": false,

lib/ansible/modules/cloud/vmware/vmware_local_role_manager.py

@@ -146,7 +146,7 @@ privileges_previous:
 local_role_name:
     description: Name of local role
     returned: always
-    type: string
+    type: str
 new_privileges:
     description: List of privileges
     returned: always

lib/ansible/modules/crypto/openssl_privatekey.py

@@ -168,7 +168,7 @@ type:
 curve:
     description: Elliptic curve used to generate the TLS/SSL private key
     returned: changed or success, and I(type) is C(ECC)
-    type: string
+    type: str
     sample: secp256k1
 filename:
     description: Path to the generated TLS/SSL private key file

lib/ansible/modules/network/netvisor/pn_admin_session_timeout.py

@@ -64,7 +64,7 @@ RETURN = """
 command:
   description: the CLI command run on the target node.
   returned: always
-  type: string
+  type: str
 stdout:
   description: set of responses from the admin-session-timeout command.
   returned: always

lib/ansible/modules/network/netvisor/pn_cpu_class.py

@@ -83,7 +83,7 @@ RETURN = """
 command:
   description: the CLI command run on the target node.
   returned: always
-  type: string
+  type: str
 stdout:
   description: set of responses from the cpu-class command.
   returned: always

lib/ansible/modules/network/netvisor/pn_dscp_map.py

@@ -63,7 +63,7 @@ RETURN = """
 command:
   description: the CLI command run on the target node.
   returned: always
-  type: string
+  type: str
 stdout:
   description: set of responses from the dscp-map command.
   returned: always