Add storage manager mixins

[gberginc]

Previously the only provider supporting the block and object storage
were Cinder and Swift managers. OpenStack provider was the one actually
integrating them for managing cloud volumes, snapshots, objects, ...

We are now extending the Amazon provider, integrating the EBS (Elastic
Block Store) and S3 (object store). This patch adds two mixins for block
and object store managers. It also alters the save inventory to allow
storage managers to support multiple storage types.

Since Amazon services will be using the same inventory structure,
corresponding save_inventory_cinder and save_inventory_swift have
been renamed to suit block and object storage.

This PR replaces #13351 based on the comments received.

@miq-bot add_label providers, providers/storage, refactoring
@miq-bot assign roliveri

[gberginc]

@roliveri This PR replaces #13351. It introduces mixins for block and object storage managers, handles the inventory saving and migrates existing cinder and swift managers to use the mixins.

Initial implementation for Amazon EBS (block storage) is available in a separate branch until we finalise this.

[roliveri]

@gberginc This looks good for the most part. I have one small change request. I think it would be better for the new mixins to reside under the storage_manager directory (and namespace). For example:

the definition of BlockStorageManagerMixin
defined in app/models/mixins/block_storage_manager_mixin.rb

Would become:

ManageIQ::Providers::StorageManager::BlockMixin
defined in: app/models/manageiq/providers/storage_manager/block_mixin.rb.

The same holds true for the ObjectStorageManagerMixin.

@jerryk55 @hsong-rh - Aside from the change requested in this comment, this looks good to me. Please review.

[jerryk55]

I double-checked to make sure there weren't any Swift- or Cinder-specific specs and apparently that is all rolled into the OpenStack provider - therefore LGTM.

[gberginc]

@roliveri I've moved mixins into app/models/manageiq/providers/storage_manager and updated the references in Swift and Cinder manager.

[miq-bot]

Checked commit xlab-si@1bbb4fc with ruby 2.2.5, rubocop 0.37.2, and haml-lint 0.16.1
8 files checked, 5 offenses detected

app/models/ems_refresh/save_inventory_block_storage.rb

• ❗ - Line 113, Col 29 - Rails/FindBy - Use find_by instead of where.first.
• ❕ - Line 11, Col 3 - Metrics/AbcSize - Assignment Branch Condition size for save_ems_block_storage_inventory is too high. [20.78/20]

app/models/ems_refresh/save_inventory_object_storage.rb

• ❗ - Line 56, Col 17 - Style/ExtraSpacing - Unnecessary spacing detected.
• ❗ - Line 56, Col 27 - Style/SpaceAroundOperators - Operator = should be surrounded by a single space.
• ❕ - Line 76, Col 121 - Metrics/LineLength - Line is too long. [131/120]

[gberginc]

@roliveri Do you have any other comments on this one? Can it be merged?