Fix the 'configuration._' not defined error when not provide '_' parameter.

There will be an error when we don't provide _ in webrunner. The instance-parameter:configuration is something like '''!py!{'full_address_list': [], '_': '{}'}'''

This commit process it by these steps:

1. Remove the !py prefix by str[4:].
2. Use double quotes replace single quotes within the str.
3. Get the dict {'full_address_list': [], '_': '{}'}
4. Get the value of the '_' key, in this case, it is a string "{}". Use json to convert it to a dict {}.

Notes:

• Use eval or ast.literal_eval can do it in 1 or 2 steps. But it is unsafe and can not process json string which contains true, false, etc.
• The test suite was passed in here with this commit: https://lab.nexedi.com/Daetalus/slapos/commit/6e15fa38f6164b91af23a2a1fd3c7ab5e9118e31 . The commit includes in here contains some formatting.

Please let me know if there has trouble to hardcode the str[4:] or there has corner case that I didn't cover.