Added a new pm2_env property to provide the full path + filename to the PID file of each process

This minor change introduces a new property 'pm_pid_path_abs' to pm2_env to contain the full path + the  actual filename to the PID file of the process.  The existing parameter, pm_pid_path, only provides the root path as defined in the JSON configuration when the process is started. Hence, if the parameter is set in JSON, like so:

    ...
    "pid_file" : "${INSTALLATION_DIR}/pids/$1-",
    ...

Then the 'pm_pid_path' property in jlist output looks this:

    ...
    "pm_pid_path" : "/usr/projects/pm2_nodetest/pids/testapp-",
    ...

The property added by this patch, 'pm_pid_path_abs', provides the full path in jlist output, i.e.:

    ...
    "pm_pid_path_abs" : "/usr/projects/pm2_nodetest/pids/testapp-0.pid",
    ...

If the above is unintended output for 'pm_pid_path', you may want to modify its output instead of merging this pull request as-is.