[COMMON] Change import-custom-cert from bash to sh

[COMMON] Change import-custom-cert from bash to sh

As part of removing GPLv3 license, policy components have moved from
bash to sh. This change is required in import-custom-certs script so
that custom certificates can be imported into components that try to
import them. Without this change, the init containers of the
components will just fail.

Change-Id: I6c5028428d4cd7c8baf3e96cb16a3cd91db57f9e
Issue-ID: POLICY-3232
Signed-off-by: a.sreekumar <ajith.sreekumar@bell.ca>
(cherry picked from commit f79b6676cfdc380e004f184a21bb969b2824c06e)