Replace local memcached usage with redis

[ryanpetrello]

Now that every AWX node has a local independent redis server, we can probably just get rid of our memcached usage (and just use redis instead).

In modern versions of AWX, memcached is used specifically for:

• a local cache for AWX settings access (see: awx.conf.settings); any time django.conf.settings is accessed (e.g., settings.TOWER_URL_BASE), we first query memcached via Django's cache framework (https://docs.djangoproject.com/en/3.0/topics/cache/) to see if the value exists in memcached (if not, we pull the value from the database and then place it in the cache)
• any time a setting value is changed via the API, we run a task on every AWX node, which removes the old value from that nodes' local memcached
• there are various other places where AWX code interacts with memcached, but it generally does so using Django's cache interface i.e., cache.get(), cache.set()
• AWX used to use memcached as a holding space for in-flight playbook event data, and for fact gathering, but we've removed this type of usage

PLAN OF ACTION

I highly suspect that all of the above interactions are implemented only through Django's cache interface (i.e., I don't think we've got code that directly accesses memcached using the python-memcached library), so replacing memcached with redis should be as simple as just swapping out the dependency and the Django caching configuration.

• investigate https://github.com/jazzband/django-redis as an alternative cache plugin, and add it to the list of dependencies in ./requirements
• remove the python-memcached library from our list of dependencies in ./requirements
• update our definition of CACHES under awx.settings to point at the redis Unix domain socket instead of memcached
• remove any and all installer code and configuration that provisions memcached containers
  • in the local Docker installer
  • in the k8s installer
• remove any integration or functional tests that test/verify memcached connectivity at startup/install time

[kdelee]

main thing I see as missing from our testing is this case:

"I update settings on Tower A and look to see if updated on Tower B"

going to check that on a cluster

[kdelee]

Allright, tried that out and adding a test to catch in future. Going to close as verified