roachprod: validate cloud providers

Previously, when calling the `create` command, it was possible to pass the same
cloud twice by specifying `-c` or `--clouds` for the same provider more than
once. There was no early validation that caught this and the resulting error
after attempting to create the cluster was confusing.

This change adds validation that warns the user, early on, a duplicate provider
has been passed.

Epic: None
Release note: None

pkg/cmd/roachprod/main.go

@@ -1395,6 +1395,18 @@ func validateAndConfigure(cmd *cobra.Command, args []string) {
 			_ = cmd.Flags().Set("arch", string(arch))
 		}
 	}
+
+	// Validate cloud providers, if set.
+	providersSet := make(map[string]struct{})
+	for _, p := range createVMOpts.VMProviders {
+		if _, ok := vm.Providers[p]; !ok {
+			printErrAndExit(fmt.Errorf("unknown cloud provider %q", p))
+		}
+		if _, ok := providersSet[p]; ok {
+			printErrAndExit(fmt.Errorf("duplicate cloud provider specified %q", p))
+		}
+		providersSet[p] = struct{}{}
+	}
 }
 
 var updateCmd = &cobra.Command{