rebrand 'plainbox ci' to 'checkbox sru ci'

This patch moves and changes three files from plainbox/plainbox/data/ to
checkbox-ng/contrib. Those files are picked up by the debian packaging
to implement the SRU CI loop that runs every day.

This is mostly an automatic change with one exception. The
externally-generated CI config file is still called (and read as)
plainbox, for compatibility.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge by tarmac [r=roadmr][bug=][author=zkrynicki]"
plainbox: re-add dependency on requests >= 1.1

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge by tarmac [r=roadmr][bug=1236377,1236505,1243610][author=zkrynicki]"