Fix unit tests: SKIP_ON_BOX_BASE_DIR_RESOLUTION

- recent pull request added a check (performed over ssh) to find the
  base-dir (e.g. resolve "~") on the box where brooklyn is installing
  software components. For unit-tests (e.g. run on buildhive) these
  can fail because the test machine does not have ssh keys set up.
  Therefore disable the base-fir resolution in all these unit tests.

- Adds BrooklynMgmtContextUnitTestSupport for test classes to extend
  where it creates app with SKIP_ON_BOX_BASE_DIR_RESOLUTION=true.

- TODO: want to refactor many many other tests to also extend
  BrooklynMgmtContextUnitTestSupport
Fix CassandraDatacenter seed tracking

- was failing CassandraDatacenterTest.testUpdatesSeedsFastishManyTimes
  every few cycles, after changing setup to use
  BrooklynMgmtContextUnitTestSupport (so guessing this exposed a
  different race condition).
- In each case, the previous guarded call to refreshSeeds should
  have already set CURRENT_SEEDS if required, and done so with
  good logging. Calling it again seemed pointless.
  But not sure what the actual race was - after removing the call it
  passed every time for me!