Fix duplicate iptables rules

If iptables version is < 1.4.11, try to delete the rule vs. checking if it exists. Fixes #6831.

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jfrazelle@users.noreply.github.com> (github: jfrazelle)
Add a little description of the image tarball format.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
'an file' -> 'a file' grammar fix

Signed-off-by: Albert Callarisa Roca <albert@acroca.com>
Merge pull request #8111 from acroca/master

Grammar fix
Merge pull request #8061 from SvenDowideit/document-image-tarball-format

Add a little description of the image tarball format.
Merge pull request #6950 from jfrazelle/6831-check-flag-centos6.5

Fix duplicate iptables rules