(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.

spec/helpers.rb

@@ -99,6 +99,10 @@ def fetch_vm(vm)
   redis.hgetall("vmpooler__vm__#{vm}")
 end
 
+def set_vm_data(vm, key, value)
+  redis.hset("vmpooler__vm__#{vm}", key, value)
+end
+
 def snapshot_revert_vm(vm, snapshot = '12345678901234567890123456789012')
   redis.sadd('vmpooler__tasks__snapshot-revert', "#{vm}:#{snapshot}")
   redis.hset("vmpooler__vm__#{vm}", "snapshot:#{snapshot}", "1")