Fix grep bug when plugin version specified

 * The jenkins::plugin fact is a comma separated string of plugins. When
   a plugin version is specified, the string to search is:
   'myplugin 1.2.3,'

   However, the last plugin in the jenkins::plugin fact doesn't end with
   a comma. This results in that plugin not being found by grep() and so
   puppet tries to install the plugin on each run.

   Adding a simple regex alternation checking for comma or end-of-string
   (,|$) fixes this.

 * Added test cases.