Added additional checks on the command used in the Exec plugin in a kubeconfig (#1013)

- Added check to see if the program being referenced in the command field of the
exec object in the User construct exists. If it doesn't an error will be raised.
If more than 1 context is selected when adding a kubeconfig then valid contexts
will be added and any with an error will not be.

Signed-off-by: Steve Richards <srichards@mirantis.com>

Co-authored-by: Steve Richards <srichards@mirantis.com>

types/command-exists.d.ts

@@ -0,0 +1,16 @@
+// Type definitions for command-exists 1.2
+// Project: https://github.com/mathisonian/command-exists
+// Definitions by: BendingBender <https://github.com/BendingBender>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+
+export = commandExists;
+
+declare function commandExists(commandName: string): Promise<string>;
+declare function commandExists(
+    commandName: string,
+    cb: (error: null, exists: boolean) => void
+): void;
+
+declare namespace commandExists {
+    function sync(commandName: string): boolean;
+}