Increase ping timeout on scenario testing

Some gating may fail because ping timeout is short (20s).
In this commit,  we will increase up to 60s.

- Added ping_timeout for compute config with default 60s
- Replaced hardcorded ssh timeout value with ssh_timeout

Fixes bug 1194026

Change-Id: If4e64aff17fc9aea1b6de03c684dff145ef5e6f2
Merge "Increase ping timeout on scenario testing"