What this PR does / why we need it:
Add "shutdown" syntax and test case.
You can stop the openGauss database node by command "shutdown [ | fast | smart | immediate ]".
There are three modes of shutdown operation as same as "gs_ctl stop", fast，smart and immediate. Default value is fast, and smart takes effect only in standby mode.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Release note: