Revert "Aliases: Throw exception if index is null when creating alias"

This reverts commit ee857bc07302b5bfcb327b3be9d07d9c6de28254.
Docs: Tidied the upgrade indices docs
Rest: Added missing parameters to indices.upgrade
Spec: Removed flush and max_num_segments from indices.upgrade
Docs: Add missing link to the 2.0 migration guide.
Tests: Add assumption to restrict upgrade test from running against
invalid backcompat versions.
Throw parsing exception if terms filter or query has more than one field

Closes #5014
Admin: Fix upgrade logic to work on lucene major version differences.

Also improved upgrade tests, and added test against static ES 0.20 index
which used Lucene 3.6.

closes #8013
Temporarily ignore 0.20 upgrade test until it can be made less flaky.
[TEST] Remove explicit network mode - not needed here
Logging: add shard id to logging in InternalIndexService.removeShard
Test: re-enable testUpgrade_0_20
[CORE] Simplify discovery node initialization if version is unknown

Today it's easy to use the wrong version when initializing DiscoveryNode
instances. This commit adds javadocs and a utility constant to
initialize DiscoveryNode instances if the the remotes node version is
unknown.

Closes #8051