erasure-code: do not hide overloaded ErasureCode::parse()

erasure-code: do not hide overloaded ErasureCode::parse()

* this change fixes the warning from clang:
  ErasureCodeJerasure::parse' hides overloaded virtual function
  [-Woverloaded-virtual]
* some erasure codecs' ErasureCode::parse() rewrites the profile
  using the default values when parsing it if the corresponding
  items are not specified. and we don't call ErasureCode::parse()
  via its children's references. so no need to make it a virtual
  function.
* and ErasureCode::parse() is used as a helper function by its
  children, so make it `protected`
* and parse() in ErasureCode's children is but a helper function
  called by ctor, descendants' parse() and init(). so make them
  protected or private accordingly.

Signed-off-by: Kefu Chai <kchai@redhat.com>