ABFS: tune imports & javadocs; stabilise tests

Followup on HADOOP-15540 with some initial review tuning

Tuning

• ordering of imports
• rely on azure-auth-keys.xml to store credentials (change imports, docs,.gitignore)
• log4j -> info
• add a "." to the first sentence of all the javadocs I noticed.
• remove @Public annotations except for some constants (which includes some commitment to maintain them).
• move the AbstractFS declarations out of the src/test/resources XML file into core-default.xml for all to use
• other IDE-suggested tweaks

Testing

Review the tests, move to ContractTestUtil assertions, make more consistent to contract test setup, and general work to make the tests work well over slower links, document, etc.