(POOLER-123) Implement a max TTL (#349)

* (POOLER-123) Implement a max TTL

Before this change, we could checkout a vm and set the lifetime to a
very high number which would esssentially keep the vm running forever.
Now implementing a config setting max_lifetime_upper_limit which enforces
a maximum lifetime in hours both for initial checkout and extending a
running vm

* (POOLER-123) Improve PUT vm endpoint error messaging

Prior to this commit the PUT vm endpoint didn't give any useful
information about why a user's request failed.

This commit updates PUT to output a more helpful set of error messages
in the `failure` key that gets returned in the JSON response.

* (POOLER-123) Update max_lifetime_upper_limit key

This commit switches the max_lifetime_upper_limit key from being a
symbol to being a string, which is what the config hash seems to contain.

* (maint) Add option to disable Redis persistence in docker-compose

This commit is just a handy little command override to the redis
container to prevent persistence.

docker/docker-compose.yml

@@ -22,6 +22,8 @@ services:
       - redislocal
   redislocal:
     image: redis
+    # Uncomment this if you don't want the redis data to persist
+    #command: "redis-server --save '' --appendonly no"
     ports:
       - "6379:6379"
     networks: