qa/workunits: cephtool: add 'expect_response' function

Uses 'check_response' but handles all the file and return code foo itself
instead of delegatig that to the user

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
qa/workunits: cephtool: use expect_response() where appropriate

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>