refactor create or get image process to eliminate reduplicate

refactor create or get image process to eliminate reduplicate

Some places when create image is referred, they check if image exist,
if exist use it directly, else create a new one.
Abstract a method to integrate the process

JIRA: FUNCTEST-432

Change-Id: I83d1112aa5cb0d3cdfab92fd49cd5f2c1cceff82
Signed-off-by: SerenaFeng <feng.xiaowei@zte.com.cn>
(cherry picked from commit 5f5af6b1d8d0af19db143e67f04f67a643af863c)