CA-111813: race condition setting "iscsi_sessions" in other_conf

This changeset basically reinstate previous code,
backing out "changeset:   1555:ac091775c8ea" in the Mercurial
repository.
To be safe, we remove the key before adding, just in case.

Signed-off-by: Germano Percossi <germano.percossi@citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>

GitHub: closes #41 on xapi-project/sm