Make issue rypes as an enumeration

Easier to copy&paste, and delete all except the correct line.

ISSUE_TEMPLATE.md

 ##### Issue Type:
 
-Can you help us out in labelling this by telling us what kind of ticket this this?  You can say “Bug Report”, “Feature Idea”, “Feature Pull Request”, “New Module Pull Request”, “Bugfix Pull Request”, “Documentation Report”, or “Docs Pull Request”.
+Can you help us out in labelling this by telling us what kind of ticket this this?  You can say:
+  - Bug Report
+  - Feature Idea
+  - Feature Pull Request
+  - New Module Pull Request
+  - Bugfix Pull Request
+  - Documentation Report
+  - Docs Pull Request
 
 ##### Ansible Version: