Check file for playbook before running syntax-check

[priyamsahoo]

The PR has the code to check a file for playbook before running --syntax-check during validation. This is to avoid errors generated by --syntax-check on non-playbook files (i.e., task files, var files, galaxy.yml files, etc.).
The content of the PR:

• A utility function to check a file for playbook
• Updated way to run ansiblePlaybook service (for --syntax-check) only if isPlaybook method return true.
• Resolves: The service ansible-playbook --syntax-check does not check file before execution #81

[ssbarnea]

Determining if a file is a playbook or another file type is far more complex than this and we should make the determined file type visible in the ui in the future. Still this change looks as good improvement of current behavior.