[COMMON] Fix bashisms in import-custom-cert

[COMMON] Fix bashisms in import-custom-cert

Bashisms of type (should be 'b = a') were all fixed previously
but a new one was reintroduced during the fixes of other types.

Also commit f79b6676cfdc380e004f184a21bb969b2824c06e moved
import-custom-cert shebang from bash to sh but substring syntaxes
similar to ${f: -4} and only supported by bash were not migrated.

Let's fix that alltogether
before enforcing the checkbashisms tox profile.

Issue-ID: OOM-2643
Issue-ID: POLICY-3232
Signed-off-by: guillaume.lambert <guillaume.lambert@orange.com>
Change-Id: Ie9b5ac1c2edd9ddf3574f09c77ca8734f2311d1d