mender: make sure that mender.conf is valid JSON
mender: make sure that mender.conf is valid JSON

Make sure that default config is valid JSON otherwise client exists with
parse error.

Signed-off-by: Maciej Borzecki <f609d9f713abcd5f8f0e51f08dc151638a7681e7@open-rnd.pl>

bboozzoo/meta-mender,bboozzoo/meta-mender,GregorioDiStefano/meta-mender,GregorioDiStefano/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender,GregorioDiStefano/meta-mender