Almost all test scripts currently need to override the __init__()
method. When they do that they need to call into super().__init__() as
the base class does some generic initialization.
This commit makes the base class __init__() call into set_test_params()
method. Individual test cases can override set_test_params() to setup
their test parameters.
Separates the act of creating a TestNode object from starting the node.
The test_framework now keeps track of its list of TestNodes, and test
writers can call start_node() and stop_node() without having to update
the self.nodes list.
Removes vchDefaultKey which was only used for first run detection.
Improves wallet first run detection by checking to see if any keys
were read from the database.
This will now also check for a valid defaultkey for backwards
compatibility reasons and to check for any corruption.
Keys will stil be generated on the first one, but there won't be
any shown in the address book as was previously done.
