clone.sh: workaround for SSH host key verification failed

Sometimes we get SSH host key verification failed error while doing
gerrit query in clone.sh [1]. This patch workaround it by SSH options
"-o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no"

[1]: https://ci.trustedfirmware.org/job/tf-a-gerrit-tforg-l2/458/console#:~:text=04%3A25%3A31-,Host%20key%20verification%20failed.,-04%3A25%3A31

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: Ib71ae9b05e470898608823b0a29b3bbb345c64c6
1 file changed