Use HTTP keep-alive feature in HTTPClient class

Currently  HTTPClient creates a new TCP connection for each
API request. We could actually reuse created connections by
relying upon HTTP keep-alive feature.  That would enable us
to do a few queries in a row more efficiently.

Fixes bug 1170385.

Change-Id: Ie6d8fb9670938e3790017509a242630b67abd794
Make list flavor show extra specs optional.

Flavor list will get each flavors extra_specs by making fresh requests to nova.
When there are lots of flavors, the flavor list will take a while to run. So
let us make show extra-specs optional.

Fix bug: #1166455

Change-Id: I86aef1035be6a88b8d9fb49a89f5a608a72589dd
Merge "Make list flavor show extra specs optional."