HeatContext model update to match heat code update

HeatContext model update to match heat code update

In Heat Liberty release OS::Nova::Server will always use the user
pre-configured in the image (e.g. "fedora" for stock Fedora cloud
images, "ubuntu" for stock Ubuntu cloud images, "cloud-user" for
stock CentOS cloud images etc)

Change all ec2-user -> ubuntu

Add admin-user in Heat model for backwards compatibility.

Refer below links for detalis:
https://etherpad.opnfv.org/p/yardstick_release_b_troubleshooting
https://github.com/openstack/heat/commit/e423bec7f10b0f5d07f05d195b3b7860f6bceb00
http://blog.scottlowe.org/2015/04/23/ubuntu-openstack-heat-cloud-init/

JIRA: -

Change-Id: I6b8b2b21daf113a3a86aee1126b0c3e74737ef4f
Signed-off-by: QiLiang <liangqi1@huawei.com>