LU-11010 tests: remove return after skip for conf-sanity

LU-11010 tests: remove return after skip for conf-sanity

The skip() routine now contains a call to exit. All calls
to skip() and skip_env() should be reviewed and calls to
return() that followed skip() should be removed.

This is the fifth patch in a series of patches that
remove calls to return() after skip() in the Lustre test
suites.

A comment is added to skip_env() defining when it should
be used.

Calls to return() after skip() are removed for:
conf-sanity.sh
functions.sh

Signed-off-by: James Nunez <jnunez@whamcloud.com>
Change-Id: Idbcdffda38aaac07f128ae42a2ffcda8986afc33
Reviewed-on: https://review.whamcloud.com/32735
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>