erasure-code: separate profile from pool creation

Instead of being hard coded, the erasure code profile is read from yaml
as:

    erasure_code_profile:
      name: teuthologyprofile
      k: 2
      m: 1
      ruleset-failure-domain: osd

Which translates into

    ceph osd erasure-code-profile set teuthologyprofile \
        k=2 m=1 ruleset-failure-domain=osd

The semantic of k/m etc. depends on the erasure code plugin, but it is
common to use k as the dividing factor for each object and m as the
number of coding chunks.

Using a different set of parameters or a different plugin can
be done without modifying the teuthology code:

    erasure_code_profile:
      name: LRCprofile
      plugin: LRC
      mapping: __DD__DD
      layers: [[ "_cDD_cDD", "" ], [ "cDDD____", "" ], [ "____cDDD", ""]]

For backward compatibility, the default erasure code profile is set to

      k: 2
      m: 1
      ruleset-failure-domain: osd

which matches the previously hardcoded default of the corresponding
functions.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #271 from dachary/wip-erasure-code-profile

erasure-code: separate profile from pool creation