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
3 files changed