Can set garbage for core experimental.layouts

[Gerry Boland]

I accidentally ran

sudo snap set core experimental.layouts=rue

instead of using "true" and the command succeeded. I only noticed the error when installing a snap using layouts:

sudo snap try ./prime --devmode
error: cannot try /home/gerry/dev/snaps/xeyes-snap/prime: internal error: cannot unmarshal snap
       "core" option "experimental.layouts" into *bool: json: cannot unmarshal string into Go value
       of type bool, json: "rue"

The value should be checked in the configure hook.

[Zygmunt Krynicki (zyga)]

This is now fixed in master.