As a user, I can mirror Galaxy roles to a local repo

[daviddavis]

fixes #3180
https://pulp.plan.io/issues/3180

[daviddavis]

Test steps:

export I=$RANDOM

http POST :8000/api/v3/repositories/ name=tester$I

export REPO_HREF=$(http :8000/api/v3/repositories/ | jq -r ".results[] | select(.name == \"tester$I\") | ._href")

http POST :8000/api/v3/importers/ansible/ name=testimporter$I download_policy='immediate' sync_mode='mirror' feed_url='https://galaxy.ansible.com/api/v1/roles/?namespace=ansible'

export IMPORTER_HREF=$(http :8000/api/v3/importers/ansible/ | jq -r ".results[] | select(.name == \"testimporter$I\") | ._href")

http POST ${IMPORTER_HREF}sync/ repository=$REPO_HREF

http ${REPO_HREF}versions/1/content/

[bmbouter]

Using this code I was able to sync 11 ansible roles from galaxy! I also did a read-through and it looks all reasonable and correct. Thanks for submitting such a great PR!