Fixes for user-locally-built rpms of td-agent

Fixes for rpm specs and Makefiles to be able to accept user-defined customized configuration files and plugin lists.

 * customized config file (td-agent.conf) acceptable as redhat/td-agent.conf.custom
   * "td-agent.conf" (renamed as td-agent.conf.tmpl later) already used
   * current "td-agent.conf" should be located in share directory, but such that change breaks compatibility of Makefile
 * customized pre-installed plugin list as lines of %configure
   * and its receiver new Makefile task
 * pre-downloaded gemfile directory (plugin_gems)
   * rubygems.org mirrors returns 503 for many-frequently-requests
   * 1 or 2 additional gems from current default list makes many of 503 errors