Add issue template for {bug,enhance,support}

.github/ISSUE_TEMPLATE/bug-report.md

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+---
+name: Bug Report
+about: Report a bug encountered
+
+---
+<!-- Please use this template while reporting a bug and provide as much info as possible. Not doing so may result in your bug not being addressed in a timely manner. Thanks!-->
+
+
+**What happend**:
+
+**What you expected to happen**:
+
+**How to reproduce it (as minimally and precisely as possible)**:
+
+**Anything else we need to know?**:
+
+**Environment**:
+
+- Multus version 
+  image path and image ID (from 'docker images')
+- Kubernetes version (use `kubectl version`):
+- Primary CNI for Kubernetes cluster:
+- OS (e.g. from /etc/os-release):
+- File of '/etc/cni/net.d/'
+- File of '/etc/cni/multus/net.d'
+- NetworkAttachment info (use `kubectl get net-attach-def -o yaml`)
+- Target pod yaml info (with annotation, use `kubectl get pod <podname> -o yaml`)
+- Other log outputs (if you use multus logging)

.github/ISSUE_TEMPLATE/enhancement.md

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+---
+name: Enhancement Request
+about: Suggest an enhancement to multus
+
+---
+<!-- Please only use this template for submitting enhancement requests -->
+
+**What would you like to be added**:
+
+**Why is this needed**:

.github/ISSUE_TEMPLATE/support.md

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+---
+name: Support Request
+about: Support request or question relating to multus-cni
+
+---