docker: add centos7 target for CentOS / Red Hat 7 support

--HG--
rename : contrib/docker/centos6 => contrib/docker/centos7
docker: use official centos5 image

I guess it didn't exist when centos5 support was introduced.
run-tests: clean up unused globals that were removed by tests-refactor series

They were demoted to instance variables by cb88d4a04f58, 1ad7aabba14e,
93511a595766, 2e1aa8c1ee37, 8e7b0f4d6ac7 and 02087bc4f143.
cmdserver: include pid of server handling requests in hello message

Because unix-mode server forks child process per connection, client does not
know the pid of the server that will handle requests.  The pid is necessary
to interrupt hung process:

 1. client connects to socket server
 2. server accepts the connection, forks, and tells pid
 3. client requests "runcommand pull"
    .. hung ..
 4. client sends SIGINT to the (forked) server
 5. server returns from I/O wait

Note that getsockopt(SO_PEERCRED) of Linux cannot be used because the server
fork()s after accept().