Collector which does a simple HTTP GET

This change implements a collector which does an HTTP GET via
python requests to fetch the metadata.

It should work with any GET-able URL, however it is designed to
work with Swift TempURLs.

Swift objects are not consistent, so the Last-Modified header is
checked for each poll and metadata is not fetched if the last
modified is not newer than the previous successful poll.

This collector will be enabled for OS::Nova::Server
software_config_transport: POLL_TEMP_URL which is available
in the Juno release of Heat. Using POLL_TEMP_URL will result
in no metadata polling load on heat, which has historically been
an issue with tripleo scalability.

Change-Id: I22155c22bdcc3c81a5e945ca5436a8f29f196528