Move get_all_tenants policy enforcement to API

Cleanup to move the enforcement of the get_all_tenants policy
enforcement to the API level. The all_tenants search option in
compute/api.py is only used to trigger policy enforcement while
the logic required by all_tenants (not specifying a project_id or
user_id filter) already resides in the servers API. The flag does
not need to be passed to the db level as it is ignored at that
level anyway.

Partially implements blueprint nova-v3-api

Change-Id: I5200c8912596ffbd32c5f7c6d74939782ff70260
Merge "Move get_all_tenants policy enforcement to API"