autogenerate doc bug reports based on DocImpact tag

DocImpact has worked well for communicating new features and code
changes that impact documentation to the doc team. However, the bulk
of the emails received were essentially just turned into bug reports.
This change allows the automated creation of those bug reports.

The code searches for potential duplicate bugs before creating a bug
to avoid creating multiple bugs per review.

Note that this is originally the page from
https://review.openstack.org/#/c/30718/

However, I'm drowning in DocImpact emails and wanted to move things
along while the original author (Steven Deaton) is on holiday.

There is potential future work here to:
1) alter the text on bug update
2) close bugs associated with abandoned patch sets
3) mark patch-merged bugs as 'confirmed'

though, as one of the main benefactors from this code, I am quite
happy to put this forward without these extras initially if it is
acceptable to our awesome infra team :)

patchset 2 updates documentation strings thanks to excellent feedback :)

fixes bug 1184845

Change-Id: Ia3a8a203314c4e527855df1623491f96da06b882
Reviewed-on: https://review.openstack.org/32348
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins