osd, tools: refactor OSDMap::calc_pg_upmaps (simplify the code)

osd, tools: refactor OSDMap::calc_pg_upmaps (simplify the code)

This is the first commit in a series of commits that aims at adding a primary balancer to Ceph and improving the current upmap balancer functionality. This first commit focuses on simplifying (refactoring) the code of `calc_pg_upmaps` so it is easier to change in the future. This PR keeps the existing functionality as-is and does not change anything but the code structure.

As part of the work is major refactoring of OSDMap::calc_pg_upmaps, the first thing is adding an --upmap-seed param to osdmaptool so test results can be compared without the random factor.

Other changes made:
    - Divided sections of `OSDMap::calc_pg_upmaps` into their own separate functions
    - Renamed tmp to tmp_osd_map
    - Changed all the occurances of 'first' and 'second' in the function to more meaningful names.

Signed-off-by: Josh Salomon <josh.salomon@gmail.com>