ceph-disk: refactor prepare

ceph-disk: refactor prepare

The logic / code path is only modified to the extent necessary for the
refactor.

The Prepare class roughly replaces the prepare_main function but also
handles the prepare subcommand argument parsing. It creates the data and
journal objects and delegate the actual work to them via the prepare()
method.

The Prepare class assumes that preparing an OSD consists on the
following phases:

  * optionally prepare auxiliary devices, such as the journal
  * prepare a data directory or device
  * populate the data directory with fsid etc. and optionally
    symbolic links to the auxiliary devices

The PrepareDefault class is derived from Prepare and implements the
current model where there only is one auxiliary device, the journal.

The PrepareJournal class implements the *journal* functions
and is based on a generic class, PrepareSpace which handles the
allocation of an auxiliary device. The only journal specific feature is
left to the PrepareJournal class: querying the OSD to figure out if
a journal is wanted or not.

The OSD data directory is prepared via the PrepareData class. It creates
a file system if necessary (i.e. if a device) and populate the data
directory. Further preparation is then delegated to the auxiliary
devices (i.e. adding a symlink to the device for a journal).

There was some code paths related dmcrypt / multipath devices in
the prepare functions, although it is orthogonal. A class tree for
Devices was created to isolate that.

Although that was the primary reason for adding a new class tree, two
other aspects have also been moved there: ptypes and partition creation.
The ptypes are organized into a data structure with a few helpers in
the hope it will be easier to maintain. All references to the *_UUID
variables have been updated.

The creation of a partition is delegated to sgdisk and a wrapper helps
reduce the code redundancy.

The ptype of a given partition depends on the type of the device (is it
dmcrypt'ed or a multipath device ?). It is best implemented by
derivation so the prepare function does not need to be concerned about
how the ptype of a partition is determined.

Many functions could be refactored into a Device class and its
derivatives, but that was not done to minimize the size of the refactor.

  Device knows how to create a partition and figure out the ptype tobe
  DevicePartition a regular device partition
  DevicePartitionMultipath a partition of a multipath device
  DevicePartitionCrypt base class for luks/plain dmcrypt, can map/unmap
  DevicePartitionCryptPlain knows how to setup dmcrypt plain
  DevicePartitionCryptLuks knows how to setup dmcrypt plain

The CryptHelpers class is introduced to factorize the code snippets that
were duplicated in various places but that do not really belong
because they are convenience wrappers to figure out:

   * if dmrypt should be used
   * the keysize
   * the dmcrypt type (plain or luks)

Signed-off-by: Loic Dachary <loic@dachary.org>