Currently the ec2 collector polls the nova metadata service every
$polling_period even though the data is not expected to change and no
known config actions have been written to respond to changes in these
values.
With larger overclouds, this metadata polling will cause noticeable
load on the undercloud, especially nova-api and neutron (for port
lookups).
This change allows collect calls to raise a SourceAlreadyCollected
exception if the data for this collector never changes. The ec2
collector raises this if its data has already been cached to
/var/lib/os-collect-config/ec2.json
Change-Id: Ib7df590601132857690c8ab64fe32098a81752d8
Closes-Bug: #1619072
8717436db9