Add constraints to check whether image/keypair exists

Add constraints for "KeyName" and "ImageId" to check whether keypair/image
exists on "AWS::AutoScaling::LaunchConfiguration" resource.

Change-Id: I23dc126e621f3cfa7ca86b3a1c27dd949d3f5093
Closes-Bug: #1312564
Use Constraint stubs in common.py

Migrate to use functions: stub_KeypairConstraint_validate() and
stub_ImageConstraint_validate() in tests/common.py which defined 
in patch I23dc126e621f3cfa7ca86b3a1c27dd949d3f5093.

Change-Id: I63d578a9291d0fe08815d6a33323324432ae21cb
Merge "Add constraints to check whether image/keypair exists"
Merge "Use Constraint stubs in common.py"