lava: Set default shell for submitted jobs

With the older lava-server installation on lava-master-02, the default
shell was changed by setting `lava_test_sh_cmd` in the base device
type template file
`/etc/lava-server/dispatcher-config/device-types/base.jinja2`.

Rather than setting the value for all devices by default in global
configuration, I think it is best to make the expected shell explicit
in our submitted job configuration. The SHELL environment variable is
read and used by the `lava_dispatcher/lava_test_shell/lava-test-shell`
script to set the interpreter for the tests.

Note: It doesn't seem possible to update the SHELL or
`lava_test_sh_cmd` on a per-test basis.

Change-Id: I7a02f1e76cfc50dfea7925d27609533e831bd221