Add show flavors command.

This pull request adds a 'show flavors' command to the `rumm` command suite.

It outputs data such as:

```
ID                Name
==                ====
2                 512MB Standard Instance
3                 1GB Standard Instance
4                 2GB Standard Instance
5                 4GB Standard Instance
6                 8GB Standard Instance
7                 15GB Standard Instance
8                 30GB Standard Instance
performance1-1    1 GB Performance
performance1-2    2 GB Performance
performance1-4    4 GB Performance
performance1-8    8 GB Performance
performance2-120  120 GB Performance
performance2-15   15 GB Performance
performance2-30   30 GB Performance
performance2-60   60 GB Performance
performance2-90   90 GB Performance
```