Revert "Update OS_IMAGES.md"

While a new os image was created, it did not contain the change that was
outlined in OS_IMAGES.md. Additionally, the centos os image was rolled
back to the previous version.

This reverts commit 8a76fa58b6e1704db9d5d13a5ef6db503903f3af.
Skip non-warden tests

Signed-off-by: Andrea Law <alaw@pivotal.io>
Explicitly chmod 755 vcap home directory

On CentOS, useradd -m sets the permissions of the home directory to 700,
which is not permissive enough for warden to stream in the agent
settings JSON.

Signed-off-by: Andrea Law <alaw@pivotallabs.com>
Don't rely on 'df' inside warden

There was a pre-existing guard in a test in with_release_stemcell_spec
to not run 'df' within warden as it may return "df: no file systems
processed".  This seems to be a known issue.

This change adds a guard around another call to 'df', done in a before
block for the aforementioned test.

Signed-off-by: Andrea Law <alaw@pivotallabs.com>