Fix modify_single_file generate empty file issue (#145)

Fix hostcfgd modify_single_file method generates empty file issue

#### Why I did it
Fix hostcfgd modify_single_file method generates empty file issue: sonic-net/sonic-buildimage#19748

In the original code, there are 2 'mv' commands to backup and copy new file, if hostcfgd crash between those 2 commands, the original file will be removed, then when hostcfgd restart later, it will generate a empty file.

The issue can be fix by use cp command.

##### Work item tracking
- Microsoft ADO: 29064569

#### How I did it
Replace 'mv' command to 'cp' command

#### How to verify it
Manually verified.
Pass all test case.

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will updated with this PR image later.
- [] SONiC.master-16482.360728-2c8b4066f

#### Description for the changelog
Fix hostcfgd modify_single_file method generates empty file issue

#### Link to config_db schema for YANG module changes
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scripts/hostcfgd

@@ -612,8 +612,8 @@ class AaaCfg(object):
             e_operations = [item for sublist in zip(e_list, operations) for item in sublist]
             with open(filename+'.new', 'w') as f:
                 subprocess.call(["sed"] + e_operations + [filename], stdout=f)
-            subprocess.call(["mv", '-f', filename, filename+'.old'])
-            subprocess.call(['mv', '-f', filename+'.new', filename])
+            subprocess.call(["cp", '-f', filename, filename+'.old'])
+            subprocess.call(['cp', '-f', filename+'.new', filename])
 
         self.check_file_not_empty(filename)