Tempest: Add support for extend attached encrypted volumes tests

We've LUKSv1 and LUKSv2 tests[1] in tempest that requires the
``extend_attached_encrypted_volume`` option to be True but
currently there is no way to set it in devstack.
This patch adds the parameter
``TEMPEST_EXTEND_ATTACHED_ENCRYPTED_VOLUME`` to enable the tests.

[1] https://github.com/openstack/tempest/blob/cb03598a65f47c51406fc86c9c1503fe42424848/tempest/api/volume/admin/test_encrypted_volumes_extend.py

Change-Id: Id3a3483629794ac38cb314812eeff84b677f35cd

Author: rajathere

lib/tempest

@@ -586,6 +586,7 @@ function configure_tempest {
         TEMPEST_VOLUME_REVERT_TO_SNAPSHOT=${TEMPEST_VOLUME_REVERT_TO_SNAPSHOT:-True}
     fi
     iniset $TEMPEST_CONFIG volume-feature-enabled volume_revert $(trueorfalse False TEMPEST_VOLUME_REVERT_TO_SNAPSHOT)
+    iniset $TEMPEST_CONFIG volume-feature-enabled extend_attached_encrypted_volume ${TEMPEST_EXTEND_ATTACHED_ENCRYPTED_VOLUME:-False}
     if [[ "$CINDER_BACKUP_DRIVER" == *"swift"* ]]; then
         iniset $TEMPEST_CONFIG volume backup_driver swift
     fi