[tune] Add number of trials to the trial runner logger

[hartikainen]

Adds the number of trials (total and per status) to the tune trial runner status log.

Example output:

Using FIFO scheduling algorithm.
Resources requested: 1/12 CPUs, 0.05/1 GPUs (1/1.0 debug-resource)
Memory usage on this node: 21.2/33.7 GB
Result logdir: ~/ray_results/gym/Point2DEnv/Wall-v0/2019-02-15T16-10-35-terminate-at-goal-test-1
Number of trials: 12 ({'RUNNING': 1, 'PENDING': 11})
PENDING trials:
 - id=851f8552-seed=258:        PENDING
 - id=904c53d5-seed=13: PENDING
 - id=fddbadf1-seed=1467:       PENDING
 - id=8e765406-seed=6687:       PENDING
 - id=7463f813-seed=2068:       PENDING
 - id=24c388d2-seed=3970:       PENDING
 - id=be17c7ea-seed=1420:       PENDING
 - id=5c0f74d7-seed=3506:       PENDING
 - id=6d420c2b-seed=1779:       PENDING
 - id=2e5df4c4-seed=7364:       PENDING
 - id=6bd5056d-seed=1902:       PENDING
RUNNING trials:
 - id=2ea69587-seed=6858:       RUNNING

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/Ray-PRB/12011/
Test PASSed.

[ericl]

saves having to awkwardly add up the number of trials :)