EMR: Update help text and examples.
1. Add helptext description, which says leaving instance-count creates one node cluster;
2. Sanitizing add-tags example
3. Edit/change example text for create-cluster and add-tags
EMR: Update modify-cluster-attributes error message.
EMR: Remove UnknownApplicationError

This fix removes the validation logic on application names passed in
"aws emr create-cluster --applications Name=<application name>"
Merge branch 'emr' into release-1.4.3
Updated the changelog for new features.
Bumping version to 1.4.3
Merge branch 'release-1.4.3' into develop

* release-1.4.3:
  Bumping version to 1.4.3
  Updated the changelog for new features.
  EMR: Remove UnknownApplicationError
  EMR: Update modify-cluster-attributes error message.
  EMR: Update help text and examples. 1. Add helptext description, which says leaving instance-count creates one node cluster; 2. Sanitizing add-tags example 3. Edit/change example text for create-cluster and add-tags