[tests] Add node methods to test framework #10171

First step towards #10082:

• moves initialize_chain() and initialize_chain_clean() in their entireties to be methods of BitcoinTestFramework. At the moment, these are only ever called by BitcoinTestFramework, so it makes sense to move them to be class methods.
• add wrapper methods for start / stop nodes functions to BitcoinTestFramework class
• [trivial] restructures ordering of members of BitcoinTestFramework for clarity.

looks reasonable. Have you been able to figure out why Travis is timing out?

Have you been able to figure out why Travis is timing out?

Because I messed up the merge :( Should be fixed now.

ACK 2558b9a25047768b955863baa858f13e1f631ef1

Makes groking test_framework simpler.

Edit: tested locally.

rebased

ACK 6291f47

Perhaps the commit message in the last commit should be fixed, though? "Reorganize BitcoinTestFramework class"

Thanks @sdaftuar . Commit message fixed. https://github.com/bitcoin/bitcoin/pull/10171/commits/4550049903608a98176d8a3f8b13ec0e00281ac5 is exactly the same as https://github.com/bitcoin/bitcoin/commit/6291f47545bfd89e73f16e8c796dd0bb51161f5a but with a new commit message.

re-ACK 4550049