In Train cycle, we moved the definition of default values
to openstack_dashboard/defaults.py. The current code accesses
a dict member using []. It requires operators to define a dict
setting with a full member.
This commit allows to use dict-type settings with partial members.
A new function is introduced to retrieve a dict-type setting
considering default values defined in
{openstack_dashboard,horizon,openstack_auth}/defaults.py
Change-Id: I7ff0ad4bca698aef9c0eba370b0570200a14367a
Closes-Bug: #1843104
ec970fd6e8