[SPARK-7699] [CORE] Lazy start the scheduler for dynamic allocation

This patch propose to lazy start the scheduler for dynamic allocation to avoid fast ramp down executor numbers is load is less.

This implementation will:
1. immediately start the scheduler is `numExecutorsTarget` is 0, this is the expected behavior.
2. if `numExecutorsTarget` is not zero, start the scheduler until the number is satisfied, if the load is less, this initial started executors will last for at least 60 seconds, user will have a window to submit a job, no need to revamp the executors.
3. if `numExecutorsTarget` is not satisfied until the timeout, this means resource is not enough, the scheduler will start until this timeout, will not wait infinitely.

Please help to review, thanks a lot.

Author: jerryshao <saisai.shao@intel.com>

Closes #6430 from jerryshao/SPARK-7699 and squashes the following commits:

02cac8e [jerryshao] Address the comments
7242450 [jerryshao] Remove the useless import
ecc0b00 [jerryshao] Address the comments
6f75f00 [jerryshao] Style changes
8b8decc [jerryshao] change the test name
fb822ca [jerryshao] Change the solution according to comments
1cc74e5 [jerryshao] Lazy start the scheduler for dynamic allocation