erasure-code: parse function for the mapping parameter

Each D letter is a data chunk. For instance:

    _DDD_DDD

is going to parse into:

   [ 1, 2, 3, 5, 6, 7 ]

the 0 and 4 positions are not used by chunks and do not show in the
mapping. Implement ErasureCode::parse to support a reasonable default
for the mapping parameter.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: rework ErasureCode*::parse methods

The ErasureCode::parse virtual function is derived in
ErasureCodeJerasure. It is reworked to share parsing arguments the
various techniques have in common. The logic is otherwise unmodified.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: ErasureCodeInterface::get_coding_chunk_count

New convenience function that returns get_chunk_count() -
get_data_chunk_count(). It is implemented in the example and in the
ErasureCode base class.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: test LRC pool, profile and crush ruleset

Using the command line to create an LRC pool validates that the plugin
can be loaded and the ruleset created immediately after the crush table
is populated with the default rulesets.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: relax profile parameter syntax

Do not restrict the character set of the key=value pairs of osd
erasure-code-profile set to allow for things like:

             layers='[
                       [ "_aAA_aAA", "" ],
                       [ "bBBB____", "" ],
                       [ "____cCCC", "" ],
                     ]'

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: locally repairable code plugin

Recursively apply erasure code techniques so that recovering from the
loss of some chunks only require a subset of the available chunks, most
of the time.

The implementation is documented in
doc/rados/operations/erasure-code-lrc.rst

http://tracker.ceph.com/issues/7238 Fixes: #7238

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