[Performance] Add a white list for nova extension to avoid API calls

[Feilong Wang]

Currently, Horizon does have a blacklist for Nova extension to skip some features of Nova which can be used by cloud operators.

However, on the Instance panel, there are 5 actions need to call the extension_supported function to check if the feature/extension is supported or not. Though it's cached, it's still taking about 2ms * 5 actions * 20 instances = 0.2 second. So it would be nice if we can have a white list for supported/known extensions to save the 0.2 second.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/511095

[Ying Zuo (yingzuo)]

I guess there's a need for saving that 0.2 second. I think it's okay to add a whitelist since we already have a blacklist. Please set a default value for OPENSTACK_NOVA_EXTENSIONS_BLACKLIST setting on local_settings to uncover this hidden gem :)

[OpenStack Infra (hudson-openstack)]

Change abandoned by Feilong Wang (<email address hidden>) on branch: master
Review: https://review.openstack.org/511095
Reason: Abandon based on feedback.