LU-15617 contrib: Add shellcheck to prepare-commit-msg

LU-15617 contrib: Add shellcheck to prepare-commit-msg

Add shellcheck verifiction to prepare-commit-msg.
SHELLCHECK_OPT export can be used to further add
option if the user wants. 'shellcheck' by deault
is set to 'yes' (SHELLCHECK_RUN="yes"). However,
it can be switched off by setting SHELLCHECK_RUN=
Tested on shellcheck version 0.8.0

Test-Parameters: trivial
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: Id0603b26202b92f68d43b82a3b2d846dd634c20e
Reviewed-on: https://review.whamcloud.com/46705
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>