tests: Avoiding a fixed 10sec sleep in test_mon_cephdf_commands()

tests: Avoiding a fixed 10sec sleep in test_mon_cephdf_commands()

The current code was waiting 10s to expect the file being put.
If the file was put in a shorter time than 10s, the test just waits for
nothing reducing the execution speed of that test.

This patch simply check if the file is actually available every second
during 10sec to exit prematurely.

This patch saves exactly 10 sec on a local system, surely a little bit
less on an infra but still saves time.
Signed-off-by: Erwan Velu <erwan@redhat.com>