Add "reboot_instance" EC2 API test

Currently, in "tempest/thirdparty/boto/test_ec2_instance_run.py",
no test case present to test the "reboot_instance" api, similar
to stop_instance, terminate_instance, etc.

Closes-Bug: #1352341

Change-Id: Ifbd8607566961dbf32b40f34ff603f712d856f6a
Merge "Add "reboot_instance" EC2 API test"