Added field to config for delimiter of multiple selections
Now uses value from multiSelectDelimiter field

Added new field to class for defining the multiselect delimiter for the field value. All I had to do was add modify the two locations where the individual values were joined.

It defaults to a comma (",").
Update ExtendedChoiceParameterDefinition.java

Wow, missed a semi-colon.
Fixed a typo.
Added help to delimiter field. Tweaked class slightly.
Merge pull request #2 from olhado/master

Added code for JENKINS-17518