Add validation of keys in config

[jcooklin]

When snapd is started with a global config file the user should receive an
error if invalid keys are present. Snapd should not start if an invalid
config is provided.

[IRCody]

I think we need a more specific definition for what makes a config invalid. Are all the sections required? If not what is the minimum set of options we are going to require? Or is it just as simple as requiring that any keys present in the config be both valid keys and in a valid position?

Some things that definitely make a config invalid:

• Invalid/unknown keys
• Valid keys in invalid positions