Fix ssh timeout issue

SSH client is using a socket timeout and an execution timeout. For
command execution, if the executed command did not generate output
within the channel timeout, test dies with TimeoutException. This is
misleading, because it wasn't the command that timed out, it just did
not generate output within channel timeout.

fixes bug 1215794

Change-Id: I90b9cb7c9cebcb78655b6c1a46f9fb09ff7deb5e
Add common "create_server_snapshot" method

There are some "create server-snapshot" methods in scenario tests,
and they will increase in the future by increasing scenario tests.
This patch adds common "create_server_snapshot" method to parent
class and uses it instead of each implementation.

Change-Id: Ib8a222d74828933c48bc6b0e8350cc072f01af4d
Merge "Add common "create_server_snapshot" method"
Merge "Fix ssh timeout issue"