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
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index a73e8a0..92371e8 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -91,6 +91,9 @@
 # IP version of the address used for SSH
 ip_version_for_ssh = 4
 
+# Number of seconds to wait to ping to an instance
+ping_timeout = 60
+
 # Number of seconds to wait to authenticate to an instance
 ssh_timeout = 300