Get LAUNCHPAD_LISTEN_FD variable from the environment

Get LAUNCHPAD_LISTEN_FD variable from the environment

Currently, the file descriptior is not generated sequentially in some devices.
The launchpad-process-pool is not always started by systemd. In this
case, the program sets LAUNCHPAD_LISTEN_FD variable to the environment.
After this patch is applied, the launchpad-process-pool tries to get the
fd from the environment if calling sd_listen_fd() is failed.

Change-Id: Ice99b52282b9ed7322acc185c150b6c8550d0d8d
Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>