Docs: add some example log lines that indicate a lack of memory when running the test suite

Docker-DCO-1.1-Signed-off-by: Jean-Baptiste Barth <jeanbaptiste.barth@gmail.com> (github: jbbarth)
Docs: improve installation/ubuntulinux memory and swap accounting section

Docker-DCO-1.1-Signed-off-by: Jean-Baptiste Barth <jeanbaptiste.barth@gmail.com> (github: jbbarth)
Merge pull request #5178 from jbbarth/docs-improve-ubuntulinux-accounting-section

Docs: improve installation/ubuntulinux memory and swap accounting section
Merge pull request #5177 from jbbarth/docs-add-common-errors-for-test-suite

Docs: add some example log lines that indicate a lack of memory when running the test suite
cli integ: don't fetch busybox if it exists

Don't make calls to the registry if the image exists already.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #5176 from unclejack/check_before_pulling

cli integ: don't fetch busybox if it exists