OSHMEM: Set SMA_SYMMETRIC_SIZE to default value

OpenSHMEMspec 1.1 introduces a set of environment variables that allows users to configure the Open-SHMEM implementation, and receive information about the implementation.
- Add SMA_SYMMETRIC_SIZE - number of bytes to allocate for symmetric heap
- SHMEM_SYMMETRIC_HEAP_SIZE (Mellanox extension) is used by a user to provide a size of symmetric area. This change sets this env variable in case a user does not set this variable
  directly.

fixed by Igor, reviewed by Miked

cmr=v1.8.2:reviwer=ompi-rm1.8