Fix `test_pod_file_based_secret` SI test
Fix `test_pod_file_based_secret` SI test

Summary: since pod volume mount path can be absolute path the test uses a relative path explicitly

Test Plan: si

Reviewers: kensipe, ichernetsky, unterstein, jeschkies, jenkins, meln1k

Reviewed By: kensipe, jeschkies, jenkins, meln1k

Subscribers: jenkins, marathon-dev, marathon-team

Differential Revision: https://phabricator.mesosphere.com/D1052

guenter/marathon,gsantovena/marathon,meln1k/marathon,janisz/marathon,guenter/marathon,mesosphere/marathon,mesosphere/marathon,meln1k/marathon,mesosphere/marathon,janisz/marathon,gsantovena/marathon,guenter/marathon,mesosphere/marathon,gsantovena/marathon,janisz/marathon,janisz/marathon,meln1k/marathon,janisz/marathon,guenter/marathon,gsantovena/marathon,meln1k/marathon,gsantovena/marathon,meln1k/marathon,mesosphere/marathon,guenter/marathon