'@' doesn't work on using '~' for home directory

[yugangw-msft]

Found it when test ACS command.
acs create -g yugangwacs2 -n yugangwacs2-1 --generate-ssh-keys --ssh-key-value @~/acs1.pub --dns-name-prefix ygacshah2

Not a blocking issue, because the command can accept a file path w/o @; however if we want to push the @ concept across, we need to use os.path.expanduser before decide it is not a valid file path.