Add support for backend defaults config group Edit

Glance provides the ability to configure multiple backends, however, there are a number of config options that needs to be repeated in each config group.
It would be beneficial to introduce a common config group "backend_defaults" (like cinder does) where we can define common config options.

Example:

Before:

[cephdriver-1]
cinder_store_project_name = service
cinder_store_password = admin
cinder_store_user_name = glance
cinder_store_auth_address = http://127.0.0.1/identity/v3
cinder_volume_type = cephdriver-1

[lvmdriver-1]
cinder_store_project_name = service
cinder_store_password = admin
cinder_store_user_name = glance
cinder_store_auth_address = http://127.0.0.1/identity/v3
cinder_volume_type = lvmdriver-1

After backend_defaults group:

[backend_defaults]
cinder_store_project_name = service
cinder_store_password = admin
cinder_store_user_name = glance
cinder_store_auth_address = http://127.0.0.1/identity/v3

[cephdriver-1]
cinder_volume_type = cephdriver-1

[lvmdriver-1]
cinder_volume_type = lvmdriver-1