AURORA-594: RC1 files missing Apache license headers

Adds missing license headers to empty and newly added files.

Testing Done:
ran: build-support/jenkins/build.sh

All test passed except test_cluster_option as outlined in AURORA-595

Bugs closed: AURORA-594

Reviewed at https://reviews.apache.org/r/23718/
AURORA-590: Release scripts branch and CHANGELOG urls are wrong in generated email template

Fixes urls in email templates to point to correct locations

Testing Done:
The CHANGELOG for the release is available at:
https://git-wip-us.apache.org/repos/asf?p=incubator-aurora.git&f=CHANGELOG&hb=0.5.0-rc1

The branch used to create the release candidate is:
https://git-wip-us.apache.org/repos/asf?p=incubator-aurora.git&hb=0.5.0-rc1

Bugs closed: AURORA-590

Reviewed at https://reviews.apache.org/r/23688/
AURORA-599 - Reset changelog and version for 0.5.0 rc2

Resets the CHANGELOG and .auroraversion back to state it was
before 0.5.0-rc1 was cut. Jira fixVersion fields have been updated
as well to match any recent resolved tickets since 0.5.0-rc1
was cut that did not get tagged correctly.

Bugs closed: AURORA-599

Reviewed at https://reviews.apache.org/r/23876/
Fix problem with deschedule command.

Bugs closed: aurora-597

Reviewed at https://reviews.apache.org/r/23872/
Updating CHANGELOG for 0.5.0 release.
Incrementing snapshot version to 0.5.1-SNAPSHOT.
Refactoring SchedulerCore (forceTaskState)

Bugs closed: AURORA-94

Reviewed at https://reviews.apache.org/r/23835/
Refactoring SchedulerCore (restartShards)

Bugs closed: AURORA-94

Reviewed at https://reviews.apache.org/r/23834/
Add an InstanceUpdater to fit into rolling update coordination in the scheduler.