Fix empty metadata issue of instance

Based on current implement, some metadata can't be extracted by
pollster, such as architecture, reservation_id, ephemeral_gb, etc.
This patch will try to get those metadata if we can and remove
those metadata what can't be pulled by pollster.

Fixes bug 1202749

Change-Id: I1f08c4eaa1cfacb612097cd0e90629d682f8acc9
Merge "Fix empty metadata issue of instance"