simplify maxtick parsing in both the python and the c++.

simplify maxtick parsing in both the python and the c++.

configs/common/Simulation.py:
    simplify maxtick code a little bit - instead of checking for -1, just set it at MaxTick.
src/python/m5/__init__.py:
    make a new m5 param called MaxTick.
src/sim/host.hh:
    fix the M5 def. of MaxTick
src/sim/main.cc:
    Simplify the MaxTick/num_cycles parsing within main.cc

--HG--
extra : convert_revision : f800addfbc1323591c2e05b892276b439b671668