the input to 'snappy config <packagename>' is expected to have
config:
<packagename>:
content:
So here we pad that input correctly. Note, that a .config file
on disk is not modified.
Also, we change 'configs' to just be 'config', to be possibly compatible
with the a future 'snappy config /' that dumped:
config:
pkg1: data1
pkg2: data2