make INITIAL_POOL_SIZE in ContainerLauncherImpl configurable to better control the thread pool size to launch/kill containers
Currently INITIAL_POOL_SIZE in ContainerLauncherImpl is hard-coded at
protected static final int INITIAL_POOL_SIZE = 10;
We should make it configurable because the thread pool size will be decided by INITIAL_POOL_SIZE, limitOnPoolSize and number of node used by the AM.
Since we already made limitOnPoolSize configurable, it make senses to also make INITIAL_POOL_SIZE configurable to better manage the thread pool size.
We saw some issue due to the small thread pool size when some node is down. The recovery from a shutdown node take very long time due to all the ContainerLauncher threads are blocked by IPC client connection to the shutdown node.