zdtm: Handle --external option in RPC
zdtm: Handle --external option in RPC

Travis sets up the GCOV variable to collect coverage and
provides an external mount for this. Thus handling this
option in RPC mode is essential.

travis-ci: success for tests: Add RPC testing to CI (rev2)
Acked-by: Andrey Vagin <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu