(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.

lib/vmpooler/api/v1.rb

@@ -875,7 +875,7 @@ module Vmpooler
       status 404
       result = { 'ok' => false }
 
-      failure = false
+      failure = []
 
       params[:hostname] = hostname_shorten(params[:hostname], config['domain'])
 
@@ -892,24 +892,42 @@ module Vmpooler
             when 'lifetime'
               need_token! if Vmpooler::API.settings.config[:auth]
 
+              # in hours, defaults to one week
+              max_lifetime_upper_limit = config['max_lifetime_upper_limit']
+              if max_lifetime_upper_limit
+                max_lifetime_upper_limit = max_lifetime_upper_limit.to_i
+                if arg.to_i >= max_lifetime_upper_limit
+                  failure.push("You provided a lifetime (#{arg}) that exceeds the configured maximum of #{max_lifetime_upper_limit}.")
+                else
+                  # also make sure we do not extend past max_lifetime_upper_limit
+                  rdata = backend.hgetall('vmpooler__vm__' + params[:hostname])
+                  running = ((Time.now - Time.parse(rdata['checkout'])) / 60 / 60).round(2)
+                  unless running + arg.to_i < max_lifetime_upper_limit
+                    failure.push("You provided a lifetime (#{arg}) that will extend the current lifetime past the configured maximum of #{max_lifetime_upper_limit}.")
+                  end
+                end
+              end
+
+              # validate lifetime is within boundaries
               unless arg.to_i > 0
-                failure = true
+                failure.push("You provided a lifetime (#{arg}) but you must provide a positive number.")
               end
             when 'tags'
               unless arg.is_a?(Hash)
-                failure = true
+                failure.push("You provided tags (#{arg}) as something other than a hash.")
               end
 
               if config['allowed_tags']
-                failure = true if not (arg.keys - config['allowed_tags']).empty?
+                failure.push("You provided unsuppored tags (#{arg}).") if not (arg.keys - config['allowed_tags']).empty?
               end
             else
-              failure = true
+              failure.push("Unknown argument #{arg}.")
           end
         end
 
-        if failure
+        if failure.size > 0
           status 400
+          result['failure'] = failure
         else
           jdata.each do |param, arg|
             case param