Migrator: added fetching shoot to minimise runtime recreation risk

[akgalwas]

Description
During the migration process users may delete runtimes. There is the following scenario:

1. I get the list of runtimes to be migrated from the KCP tool.
2. I start the migration process. Migrator in the fist step fetches all the shoots
3. The user triggers deletion of a specific runtime R1
4. Migrator creates Runtime CR for runtime R1 and applies it on the KCP
5. The shoot corresponding to runtime R1 gets deleted
6. I proceed with the rollout steps, and edit Runtime CR R1 to be controlled by the KIM.
7. The shoot gets recreated.

It is depicted on the following diagram:

There are two possible solutions to this problem:

• Using shoot, and runtimeID pair as input
• Fetching shoot from the Gardener prior to calling conversion to Runtime CR

The second option is easier to implement, and requires less changes in documentation. In addition we should check if the deletion is in progress.

Changes proposed in this pull request:

• Migrator fetches shoot to minimise the threat of recreating Runtimes that were already deleted
• Matcher fix to not detect unimportant difference
• Shoot comparator uses newest image
• Code fetching GardenerCluster CR simplified
• Error messages fixes

[m00g3n]

LGTM