If a gabbi file sets a default microversion by setting a header
'OpenStack-API-Version' with a value like 'placement latest' and then
later overrides that in an individual test with a header of
'openstack-api-version' the difference in case can lead to failure.
In the best case the failure is consistent.
In the worst case it can sometimes work, because the header shows up
twice in the request, and the last header wins, order in the headers
and the resulting list dependent on the vagrancies of python ordering.
The solution is to always use the same case, so this change updates
all use to be lowercase, to establish a precedent that future people
will be able to use as an example.
Note that gabbi is case sensitive here in part because of the
implementation but also because it provides the control and possibility
to test exactly this problem.
Change-Id: I1e89e231cf0d46d211d360cda091b33520f85027
Closes-Bug: #1728934
f974e3c356