gk-deploy: replace legacy use of 'expr index' by proper shell code
gk-deploy: replace legacy use of 'expr index' by proper shell code

pointed out by ShellCheck

Signed-off-by: Michael Adam <dcf2bf3ff8ea93047d51252815aac9756207ea09@redhat.com>

gluster/gluster-kubernetes,gluster/gluster-kubernetes,ramkrsna/gluster-kubernetes,ramkrsna/gluster-kubernetes