Undercloud installation fails when using a hostname different than 'undercloud'

Description of problem:
Undercloud installation fails when using a hostname different than 'undercloud'.

The failure reason is because the container images do not get uploaded to the local undercloud registry so the deployment fails not being able to find the images.

After https://review.openstack.org/#/c/581917/ landed the container image prepare steps get run in external_deploy_tasks but we can see in the logs that the external tasks get skipped with no hosts matched

2018-07-31 13:12:19.807 5539 WARNING tripleoclient.v1.tripleo_deploy.Deploy [ ] PLAY [External deployment step 1] *********************************************************************************************************************************************************************************
2018-07-31 13:12:19.807 5539 WARNING tripleoclient.v1.tripleo_deploy.Deploy [ ] skipping: no hosts matched

Checking the inventory we can see:

Undercloud:
  hosts:
    undercloud75:
      ansible_host: 192.168.0.1
      ctlplane_ip: 192.168.0.1
      deploy_server_id: 57075f39-3917-4660-b667-fa661305f19c
      enabled_networks: [management, storage, ctlplane, external, internal_api, storage_mgmt,
        tenant]
      external_ip: 192.168.0.1
      internal_api_ip: 192.168.0.1
      management_ip: 192.168.0.1
      storage_ip: 192.168.0.1
      storage_mgmt_ip: 192.168.0.1
      tenant_ip: 192.168.0.1
  vars: {ansible_connection: local, ansible_ssh_user: root, bootstrap_server_id: 57075f39-3917-4660-b667-fa661305f19c,
    tripleo_role_name: Undercloud}

so the playbook which runs the external_deploy_steps_tasks.yaml fails because the inventory doesn't contain an 'undercloud' host but'undercloud75' which is the undercloud machine hostname(undercloud75.localdomain) and the 'Undercloud' group(note the capital U)

- hosts: undercloud
  name: External deployment step 1
  gather_facts: no
  any_errors_fatal: yes
  become: false
  vars:
    step: '1'
  tasks:
    - import_tasks: external_deploy_steps_tasks.yaml
  tags:
    - external
    - external_deploy_steps

We can test the inventory by:

[stack@undercloud75 undercloud-ansible-AV9shq]$ ansible -i inventory.yaml -m ping undercloud
 [WARNING]: Could not match supplied host pattern, ignoring: undercloud

 [WARNING]: No hosts matched, nothing to do

[stack@undercloud75 undercloud-ansible-AV9shq]$ ansible -i inventory.yaml -m ping undercloud75
undercloud75 | SUCCESS => {
    "changed": false,
    "ping": "pong"
}
[stack@undercloud75 undercloud-ansible-AV9shq]$ vi inventory.yaml
[stack@undercloud75 undercloud-ansible-AV9shq]$ ansible -i inventory.yaml -m ping Undercloud
undercloud75 | SUCCESS => {
    "changed": false,
    "ping": "pong"
}
[stack@undercloud75 undercloud-ansible-AV9shq]$ ansible -i inventory.yaml -m ping Undercloud
vundercloud75 | SUCCESS => {
    "changed": false,
    "ping": "pong"
}

Version-Release number of selected component (if applicable):
openstack-tripleo-heat-templates-9.0.0-0.20180726103746.5fefd0b.el7ost.noarch

How reproducible:
100%

Steps to Reproduce:
1. Deploy containerized undercloud(with uploading images to local undercloud registry) on a machine with hostname != 'undercloud'

Actual results:
Deployment fails because of missing images.

Expected results:
Deployment succeeds.

Additional info: