SDD: Change tower default

CoolingTowerVariableSpeed::DesignInletAirWetBulbTemperature default is now 20,
the EnergyPlus minimum.  Normally this value will be derived from SDD by
computing design supply temp - wet bulb approach.  There are times when
that calculation will fall below the 20C minimum imposed by E+, and
OpenStudio will revert to the default 25C.  This change will establish
the floor at 20C even if the SDD requests something lower.
Merge branch 'develop' of https://github.com/NREL/OpenStudio into develop