Add support to custom memory

Add support for setting containers memory for each app from command line.
This can be disabled by setting `docker:allow-memory-set` to `false`

Config changes:
```
docker:
   memory: 256
   allow-memory-set: true
```

`docker:memory` is default memory limit for each app if not specified in command line. can be empty for no limit
`docker:allow-memory-set` enable or disabled setting custom memory limit from command line.

Command line example: `tsuru app-create myapp php 512` This creates a new app called `myapp` using platform `php` with `512`MB of memory limit for each container.