Fix command in prepare env to execute check_os.sh
Fix command in prepare env to execute check_os.sh

[1] introduced a bug when adding "sh" to the call.
There is no need to use sh or bash in the command string.

[1]: https://gerrit.opnfv.org/gerrit/#/c/36163/

Change-Id: I9ce443721b69945a4a749da0caa6c008cc8ca284
Signed-off-by: jose.lausuch <a4ed9890ad7f6494ac84a749f29a4dc3449a1186@ericsson.com>

opnfv/functest,mywulin/functest,opnfv/functest,mywulin/functest