versioning: plugins report a list of supported versions

Further progress on versioning support (Issue #266).
Bump CNI spec version to 0.3.0

SPEC.md

@@ -64,8 +64,14 @@ The operations that the CNI plugin needs to support are:
 
 - Report version
   - Parameters: NONE.
-  - Result:
-    - The version of the CNI spec implemented by the plugin: `{ "cniVersion": "0.2.0" }`
+  - Result: information about the CNI spec versions supported by the plugin
+
+      ```
+      {
+        "cniVersion": "0.3.0", // the version of the CNI spec in use for this output
+        "supportedVersions": [ "0.1.0", "0.2.0", "0.3.0" ] // the list of CNI spec versions that this plugin supports
+      }
+      ```
 
 The executable command-line API uses the type of network (see [Network Configuration](#network-configuration) below) as the name of the executable to invoke.
 It will then look for this executable in a list of predefined directories. Once found, it will invoke the executable using the following environment variables for argument passing:
@@ -85,7 +91,7 @@ Success is indicated by a return code of zero and the following JSON printed to
 
 ```
 {
-  "cniVersion": "0.2.0",
+  "cniVersion": "0.3.0",
   "ip4": {
     "ip": <ipv4-and-subnet-in-CIDR>,
     "gateway": <ipv4-of-the-gateway>,  (optional)
@@ -114,7 +120,7 @@ Examples include generating an `/etc/resolv.conf` file to be injected into the c
 Errors are indicated by a non-zero return code and the following JSON being printed to stdout:
 ```
 {
-  "cniVersion": "0.2.0",
+  "cniVersion": "0.3.0",
   "code": <numeric-error-code>,
   "msg": <short-error-message>,
   "details": <long-error-message> (optional)
@@ -151,7 +157,7 @@ Plugins may define additional fields that they accept and may generate an error
 
 ```json
 {
-  "cniVersion": "0.2.0",
+  "cniVersion": "0.3.0",
   "name": "dbnet",
   "type": "bridge",
   // type (plugin) specific
@@ -170,7 +176,7 @@ Plugins may define additional fields that they accept and may generate an error
 
 ```json
 {
-  "cniVersion": "0.2.0",
+  "cniVersion": "0.3.0",
   "name": "pci",
   "type": "ovs",
   // type (plugin) specific
@@ -220,7 +226,7 @@ Success is indicated by a zero return code and the following JSON being printed
 
 ```
 {
-  "cniVersion": "0.2.0",
+  "cniVersion": "0.3.0",
   "ip4": {
     "ip": <ipv4-and-subnet-in-CIDR>,
     "gateway": <ipv4-of-the-gateway>,  (optional)