#!/bin/sh and #!/bin/bash in kea sources

looks like we can't decide which shell Kea should use it's script.

Most of them uses !#/bin/sh but some like e.g. tools/sysrepo_config_defines.sh.sample tools/cql_config_defines.sh.sample went with !#/bin/bash.

My proposal is change all scripts to use #!/bin/sh and keep it that way in the future.