Don't validate joblog:cached-response values

We constantly get jsonschema.ValidationError on the joblog:cached-response key, because it doesn't follow the structure of other joblogs:

{"errors":[],"failed":[],"pending":[],"warnings":["'Returning cached interface info data for mx1.gen.ch.geant.net'","'error loading interface-info data from mx1.gen.ch.geant.net'","'no cached interface info for intinfo:srx2.am.office.geant.net'","'error loading interface-info data from srx2.am.office.geant.net'","'error loading interface-info data from srx2.ch.office.geant.net'","'no cached interface info for intinfo:srx2.ch.office.geant.net'","'Returning cached interface info data for qfx.par.fr.geant.net'","'error loading interface-info data from qfx.par.fr.geant.net'","'error loading interface-info data from srx1.ch.office.geant.net'","'no cached interface info for intinfo:srx1.ch.office.geant.net'"]}

requested review from @Robert.Latta

assigned to @erik.reid

see https://glitchtip.int.geant.org/python-swd-prod/issues/4?project=7