Allow for installing of required roles when using ansible-pull

[MrMMorris]

I have required roles in requirements.txt:

- src: https://github.com/picotrading/ansible-ulimit
  name: ulimit

Ansible-pull has no way of being able to run ansible-galaxy install -r requirements.yml which means in order to run any role where the dependancies are installed with requirements.txt, I have to store them in my ansible repo which is a PITA.

Maybe add a flag like -r requirements.txt that tells ansible pull to look for a file in the repo root that contains roles to be installed before running the playbook?

[bcoca]

closing as dupe of ansible/proposals#7