utils_conn: add more comments and make tmp_dir unique.

* Remove tmp_dir from param of __init__, and build tmp_dir
  using tempfile to assure it unique for each instance.
* Add more comment for the varaibles of connection class to
  make it more clear.
* Pick the CA key build method out from TLSConnection as a
  function in utils_conn.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
utils_conn: Add more unittest for utils_conn.

* Add a unittest for tmp_dir to make sure it is unique for
  each instance.
* Add three unittests for functions in utils_conn which build
  CA key.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>