Fix unsubstituted filename creation

Since merge_config_file() tries to create an unsubstituted config file,
stack.sh fails due to permission denied in the file creation when you
use '[[post-config|/$Q_PLUGIN_CONF_FILE]]' and the file does not exist.

This patch deletes unnecessary 'touch' command, because the file will
be made by 'iniset' function in the next command line with evaled
string from 'configfile'.

This patch also fixes merge_config_group() to use evaled 'configfile'
when it checks the directory of the config file exists.

Change-Id: I608847a34143b5c6a1708c180186dd88a32dd44b
Closes-bug: #1294213
(cherry picked from commit 410f5c0016a9d3b1fbd42b95ce1402a1c614e3d3)