App migration between pools

[guestisp]

Based on #1011 and #1012 an app should be migrated across pool.
Let's assume an app created on sata_pool, we should be able to migrate it over a different pool, for example, an ssd pool:

tsuru app-migrade oldpool newpool

Differences between migration and cloning:

Clone: a new app, with a new name, is created based on the old app. All is preserved (storage, file, team, users, and so on)

Migration: like cloning, but with the same name preserved and the old app is automatically destroyed when migration is done.

[tarsisazevedo]

Done in tsuru/tsuru-client@23d65cc and 9ff7dac