Fix/improve projects_yaml_alphabetized script

* use 'if ! command; then' condition to check diff
* previously "diff projects_list projects_list.sorted > projects_list.diff"
  command returns exit code 1 and due to the 'bash -xe' it fails execution
* make sort case insensitive

Now this script prints something like:

+ sed -e '/^- project: /!d' -e 's/^- project: //'
modules/openstack_project/templates/review.projects.yaml.erb
+ LC_ALL=C
+ sort projects_list -o projects_list.sorted
+ diff projects_list projects_list.sorted
+ echo 'The following projects should be alphabetized: '
The following projects should be alphabetized:
+ cat projects_list.diff
+ grep -e '> '
> openstack/openstack-doc-tools
> stackforge/cliff
> stackforge/entropy
> stackforge/fuel-devops
> stackforge/MRaaS
> stackforge/puppet_openstack_builder
> stackforge/stevedore
> stackforge/tooz
+ exit 1

Change-Id: Ib6160dc401a70ada16fa961ae8d941b3e8845d4b