Make resp_body optional on RestClientException
Make resp_body optional on RestClientException

The commit Ie1bb259e7e683081f0ad127617acc8deb98467c0 adds resp_body
to RestClientException, but it broke some unit tests of Tempest
because NotFound calls don't pass the resp_body.
So the above commit was backwards incompatible changes, this patch
makes resp_body optional for avoiding affecting to the other projects.

Change-Id: Ic75f0673f27fb23c2784f2f8dbceab70061d3823

sorenh/tempest-lib,varunarya10/tempest-lib,JioCloud/tempest-lib,cneill/tempest-lib