Grandstream template updates to support plantronics EHS via grandstream_headset_type and grandstream_headset_key_mode variables.
Also updated wallpaper and screensaver if/else statements to leave out the variables if they are not defined instead of clobbering the user setting. If you want to mandate a certain setting, just set the variable. If the variable is unset it will not overwrite on provision and allow the previous setting to remain.
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