use one random seed, and allow the user reset it

[erwincoumans]

See https://bitbucket.org/osrf/gazebo/src/32b141d9bc460970ee6c2dee8a8d6c22642cf93c/gazebo/physics/bullet/BulletPhysics.cc?at=default#cl-884

/////////////////////////////////////////////////
void BulletPhysics::SetSeed(uint32_t /_seed/)
{
// GEN_srand is defined in btRandom.h, but nothing in bullet uses it
// GEN_srand(_seed);

// The best bet is probably btSequentialImpulseConstraintSolver::setRandSeed,
// but it's not a static function.
// There's 2 other instances of random number generation in bullet classes:
// btSoftBody.cpp:1160
// btConvexHullComputer.cpp:2188
// It's going to be blank for now.
/// \todo Implement this function.
}