Correctly set use_keys when key_file is set

use_keys needs to be True for netmiko to attempt public key
authentication with the specified key_file.

Change-Id: Ic0b2c921af91e667dd95fa174a3e47ddaf37f438
(cherry picked from commit dc96c86)

Author: steveb

devstack/plugin.sh

@@ -183,6 +183,7 @@ function add_generic_switch_to_ml2_config {
 
     if [[ -n "$key_file" ]]; then
         populate_ml2_config $GENERIC_SWITCH_INI_FILE $switch_name key_file=$key_file
+        populate_ml2_config $GENERIC_SWITCH_INI_FILE $switch_name use_keys=True
     elif [[ -n "$password" ]]; then
         populate_ml2_config $GENERIC_SWITCH_INI_FILE $switch_name password=$password
     fi

doc/source/configuration.rst

@@ -14,6 +14,7 @@ Switch configuration format::
     port = <ssh port>
     username = <credential username>
     password = <credential password>
+    use_keys = <set to True when key_file is set>
     key_file = <ssh key file>
     secret = <enable secret>
     ngs_allowed_vlans = <comma-separated list of allowed vlans for switch>