fix(shell): update help to claim that policy in restore_app is optional

[neverchanje]

What problem does this PR solve?

>>> restore_app 
USAGE:  restore_app             <-c|--old_cluster_name str> <-p|--old_policy_name str>
                                <-a|--old_app_name str> <-i|--old_app_id id>
                                <-t|--timestamp/backup_id timestamp> <-b|--backup_provider_type str>
                                [-n|--new_app_name str] [-s|--skip_bad_partition]

<-p|--old_policy_name str> is required at least in the help info of restore_app command. Users are prone to make mistakes to add this argument when they restore a backup, which was created by the latest Pegasus version. https://github.com/apache/incubator-pegasus/pull/707

What it is now:

>>> restore_app 
invalid parameter
USAGE: 	restore_app             <-c|--old_cluster_name str> <-a|--old_app_name str>
	                        <-i|--old_app_id id> <-t|--timestamp/backup_id timestamp>
	                        <-b|--backup_provider_type str> [-p|--old_policy_name str]
	                        [-n|--new_app_name str] [-s|--skip_bad_partition]

--old_policy_name is not required anymore.

Checklist

Tests

• Manual test (add detailed scripts or steps below)

[acelyc111]

@neverchanje rdsn need update