Call cfn-create-aws-symlinks in boothook.sh

Images which have heat-cfntools installed from rpm or deb
will not have cfn tool links in /opt/aws/bin.

This change runs cfn-create-aws-symlinks during cloud-init
boothook.sh. It should do the following:
* if no cfn tools exist in /opt/aws/bin, symlinks from /usr/bin
  will be created
* if cfn tools exist in /opt/aws/bin, no symlinks are created
* if cfn-create-aws-symlinks doesn't exist, there will be no effect

This is required to use a vanilla Fedora 20 cloud image with heat,
which has heat-cfntools pre-installed.

Change-Id: I5426fdad3031f91b4b9c1f9c9c19ac9cc42cf2dd
Merge "Call cfn-create-aws-symlinks in boothook.sh"