blueprint host-aggregates: improvements and clean-up

This changeset addresses a number of issues found during testing:

- avoid name conflicts during aggregate creation (see db/* changes)
- avoid masking of XenAPI.Failure if pool-join fails (see plugins/* changes)
- preserve VM placement decisions made during scheduling (see xenapi/vmops.py)
- ensure plugins are called on the right hosts in XS pools (see xenapi_con.py)
- stores master uuid in aggregate metadata for use in VM live migration and
  raise InvalidAction rather than Aggregate error if we attempt to remove
  a mster (see xenapi/pool.py and compute/manager.py)
- clean-up of unit tests

Change-Id: I881a94d87efe1e81bd4f86667e75f5cbee50ce91

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