No repo master chosen if --limit in playbook invocation

When running the repo-build.yml playbook, the group_by task fails to include any repo hosts if a --limit of any kind has been specified on the playbook invocation. I excluded a compute node that was down for hardware failure when running all the playbooks for my OSA install, and this resulted in no repo master being selected - but no tasks failed as a result.

When running the rolled up setup-everything.yml or setup-infrastructure.yml playbooks, this can result in later plays failing because the repo never got built. The repo master selection task should fail loudly if no master can be selected.

Example playbook output follows:

/usr/local/bin/openstack-ansible ./repo-build.yml --limit='!compute0508,!compute0509' -v
Variable files: "-e @/etc/openstack_deploy/user_arbutus_variables.yml -e @/etc/openstack_deploy/user_secrets.yml -e @/etc/openstack_deploy/user_variables.yml "
No config file found; using defaults
 [WARNING]: Unable to parse /etc/openstack_deploy/inventory.ini as an inventory source

[DEPRECATION WARNING]: The use of 'include' for tasks has been deprecated. Use 'import_tasks' for static inclusions or 'include_tasks' for dynamic inclusions. This feature will
be removed in a future release. Deprecation warnings can be disabled by setting deprecation_warnings=False in ansible.cfg.
[DEPRECATION WARNING]: include is kept for backwards compatibility but usage is discouraged. The module documentation details page may explain more about this rationale.. This
feature will be removed in a future release. Deprecation warnings can be disabled by setting deprecation_warnings=False in ansible.cfg.
[DEPRECATION WARNING]: The use of 'static' has been deprecated. Use 'import_tasks' for static inclusion, or 'include_tasks' for dynamic inclusion. This feature will be removed
in a future release. Deprecation warnings can be disabled by setting deprecation_warnings=False in ansible.cfg.

PLAY [Group repo servers by architecture] ****************************************************************************************************************************************

TASK [Group repo servers by architecture and os version] *************************************************************************************************************************
Monday 24 September 2018 00:24:03 +0000 (0:00:00.128) 0:00:00.128 ******
ok: [lb201_repo_container-effde309] => {"add_group": "repo_servers_7.5.1804_x86_64", "changed": false, "failed": false, "parent_groups": ["all"]}

PLAY [Group repo servers by architecture] ****************************************************************************************************************************************

TASK [Group repo servers by architecture and os version] *************************************************************************************************************************
Monday 24 September 2018 00:24:03 +0000 (0:00:00.068) 0:00:00.197 ******
ok: [lb101_repo_container-06ef7f23] => {"add_group": "repo_servers_7.5.1804_x86_64", "changed": false, "failed": false, "parent_groups": ["all"]}

PLAY [Prepare group of master repo servers] **************************************************************************************************************************************
skipping: no hosts matched
 [WARNING]: Could not match supplied host pattern, ignoring: repo_masters

PLAY [Build new repo packages for a given release] *******************************************************************************************************************************
skipping: no hosts matched

PLAY RECAP ***********************************************************************************************************************************************************************
lb101_repo_container-06ef7f23 : ok=1 changed=0 unreachable=0 failed=0
lb201_repo_container-effde309 : ok=1 changed=0 unreachable=0 failed=0