Mythbuntu bare causes MCC to throw an error on startup

[Impact]
When starting mythbuntu-control-centre an error is thrown "Exception in applyStateToGUI of plugin Plugins. Disabling Plugin.". This is because we're trying to read a config file that may not exist. This is resolved by checking if the config file exists before reading it.

[Test Case]
Open mythbuntu-control-centre with an unpatched mythbuntu-bare-client installed via command line and note the error "configparser.NoSectionError: No section: 'General'"

IMPORTANT: There is a second error you may see on the command line "AttributeError: 'MythPluginsPlugin' object has no attribute 'mythweb_password_combobox'"
 that is due to bug LP:1581971 which is caused in another package and will throw the same GUI error until patched.

[Regression Potential]
Low. We're checking if the file exists prior to reading it now. In the event the file doesn't exist, we still set the defaults as before (we just don't try to read something that doesn't exist). This is pretty self contained inside mythbuntu-control-centre, so any regressions would be limited to that tab