Switch Redis Config volatile-lru to allkeys-lru

https://github.com/EasyEngine/easyengine/issues/677

volatile-lru is a poor choice as it depends on the keys stored to be explicitly marked as purgable, something that the WordPress Redis plugin DOES NOT do. This means that an EE server running Redis will fill the cache, never to clean it unless it's purged via the Nginx plugin.

Setting it to allkeys-lru ensures that the oldest used keys are always purged if out of RAM instead of segfaulting and massively slowing down a WordPress site.

ee/cli/plugins/stack.py

@@ -1907,15 +1907,15 @@ class EEStackController(CementBaseController):
             if 'redis-server' in apt_packages:
                 # set redis.conf parameter
                 # set maxmemory 10% for ram below 512MB and 20% for others
-                # set maxmemory-policy volatile-lru
+                # set maxmemory-policy allkeys-lru
                 if os.path.isfile("/etc/redis/redis.conf"):
                     if EEVariables.ee_ram < 512:
                         Log.debug(self, "Setting maxmemory variable to {0} in redis.conf"
                                             .format(int(EEVariables.ee_ram*1024*1024*0.1)))
                         EEShellExec.cmd_exec(self, "sed -i 's/# maxmemory <bytes>/maxmemory {0}/' /etc/redis/redis.conf"
                                              .format(int(EEVariables.ee_ram*1024*1024*0.1)))
-                        Log.debug(self, "Setting maxmemory-policy variable to volatile-lru in redis.conf")
-                        EEShellExec.cmd_exec(self, "sed -i 's/# maxmemory-policy.*/maxmemory-policy volatile-lru/' "
+                        Log.debug(self, "Setting maxmemory-policy variable to allkeys-lru in redis.conf")
+                        EEShellExec.cmd_exec(self, "sed -i 's/# maxmemory-policy.*/maxmemory-policy allkeys-lru/' "
                                                    "/etc/redis/redis.conf")
                         EEService.restart_service(self, 'redis-server')
                     else:
@@ -1923,8 +1923,8 @@ class EEStackController(CementBaseController):
                                             .format(int(EEVariables.ee_ram*1024*1024*0.2)))
                         EEShellExec.cmd_exec(self, "sed -i 's/# maxmemory <bytes>/maxmemory {0}/' /etc/redis/redis.conf"
                                              .format(int(EEVariables.ee_ram*1024*1024*0.2)))
-                        Log.debug(self, "Setting maxmemory-policy variable to volatile-lru in redis.conf")
-                        EEShellExec.cmd_exec(self, "sed -i 's/# maxmemory-policy.*/maxmemory-policy volatile-lru/' "
+                        Log.debug(self, "Setting maxmemory-policy variable to allkeys-lru in redis.conf")
+                        EEShellExec.cmd_exec(self, "sed -i 's/# maxmemory-policy.*/maxmemory-policy allkeys-lru/' "
                                                    "/etc/redis/redis.conf")
                         EEService.restart_service(self, 'redis-server')
             if disp_msg: