Add trusted_image_certificates to REST API

This change adds support for the trusted_image_certificates parameter,
which is used to define a list of trusted certificate IDs that can be
used during image signature verification and certificate validation. The
parameter may contain a list of strings, each string representing the ID
of a trusted certificate. The list is restricted to a maximum of 50 IDs.
The list of certificate IDs will be stored in the trusted_certs field of
the instance InstanceExtra and will be used to verify the validity of
the signing certificate of a signed instance image.

The trusted_image_certificates request parameter can be passed to
the server create and rebuild APIs (if allowed by policy):

* POST /servers
* POST /servers/{server_id}/action (rebuild)

The following policy rules were added to restrict the usage of the
``trusted_image_certificates`` request parameter in the server create
and rebuild APIs:

* os_compute_api:servers:create:trusted_certs
* os_compute_api:servers:rebuild:trusted_certs

The trusted_image_certificates parameter will be in the response
body of the following APIs (not restricted by policy):

* GET /servers/detail
* GET /servers/{server_id}
* PUT /servers/{server_id}
* POST /servers/{server_id}/action (rebuild)

APIImpact

Implements blueprint: nova-validate-certificates
Change-Id: Iedd3fea0e86648fae364f075915555dcb2c4f199

api-ref/source/servers-actions.inc

@@ -488,10 +488,11 @@ Request
   - description: server_description
   - key_name: key_name_rebuild_req
   - user_data: user_data_rebuild_req
+  - trusted_image_certificates: server_trusted_image_certificates_rebuild_req
 
-**Example Rebuild Server (rebuild Action) (v2.54)**
+**Example Rebuild Server (rebuild Action) (v2.63)**
 
-.. literalinclude:: ../../doc/api_samples/servers/v2.54/server-action-rebuild.json
+.. literalinclude:: ../../doc/api_samples/servers/v2.63/server-action-rebuild.json
    :language: javascript
 
 Response
@@ -537,10 +538,11 @@ Response
   - tags: tags
   - key_name: key_name_rebuild_resp
   - user_data: user_data_rebuild_resp
+  - trusted_image_certificates: server_trusted_image_certificates_resp
 
-**Example Rebuild Server (rebuild Action) (v2.54)**
+**Example Rebuild Server (rebuild Action) (v2.63)**
 
-.. literalinclude:: ../../doc/api_samples/servers/v2.54/server-action-rebuild-resp.json
+.. literalinclude:: ../../doc/api_samples/servers/v2.63/server-action-rebuild-resp.json
    :language: javascript
 
 Remove (Disassociate) Floating Ip (removeFloatingIp Action) (DEPRECATED)