Revert "Update Ceilometer alembic requirement to 0.6.0 version"

This reverts commit 0b3819b4a58dde35f8446ac3f76ecf8aa04c25ef.

This change was originally motivated entirely by usage of the new 'type_'
kwarg in calls to op.drop_constraint() from two ceilometer migration scripts,
even though the old 'type' form was retained for backward compatability:

  https://alembic.readthedocs.org/en/latest/ops.html

  "Changed in version 0.5.0: The type parameter is now named type_.
  The old name type will remain for backwards compatibility."

To avoid needlessly imposing the alembic 6.0 on distros for which this
version is not yet packaged, I propose to revert these migrations to use
the older form in ceilometer:

  https://review.openstack.org/48069

The global requirement may be similiarly reverted.

Change-Id: I71d5ca9aaf4789817ca2e0cf3f90a6ef8b63acd1
Merge "Revert "Update Ceilometer alembic requirement to 0.6.0 version""