Validate resource attributes for preallocated hosts before executing leases

Summary: This prevents issues when the user hasn't provided the appropriate attributes for a preallocated host.

Test Plan: Attempted to lease against a resource with omitted attributes, got an exception thrown before any SSH commands occurred.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7695
Update preallocated hosts to use Passphrase credentials

Summary: Depends on D7695.  This updates preallocated hosts to use Passphrase credentials.  Due to the way SSH private key text credentials work (the TempFile disappears before SSH commands can be executed), this only supports file-based private keys at the moment.

Test Plan:
Created a Passphrase credential for a file-based SSH key.  Allocated a resource with:

```
bin/drydock create-resource --blueprint 1 --name "My Linux Host" --attributes platform=linux,host=localhost,port=22,path=/var/drydock,credential=2
```

and successfully leased it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4111, T1049

Differential Revision: https://secure.phabricator.com/D7697
Support text-based private key credentials in DrydockSSHCommandInterface

Summary: This updates DrydockSSHCommandInterface to correctly hold open the private key credentials for the life of the interface so that remote commands will execute correctly with a text-based private key.

Test Plan: Created a text-based private key, created a resource based on it and leased against it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4111

Differential Revision: https://secure.phabricator.com/D7700
Don't try to download diffs-of-diffs

Summary:
Ref T1715. When the user clicks "Download Raw Diff" in Differential, we try to build a diff of exactly what they're seeing. However:

  - This doesn't work if any of the changes have multiple hunks, and fixing it seems hard.
  - I suspect this diff is never actually useful anyway? And probably kind of confusing in the best case. You can't really apply it to anyhting, since you'd have to apply another diff first.

Instead, just build the right-side diff, which should align well with user expectation and doesn't suffer from the multi-hunk bug.

Some day, we could maybe add some of the fancy options in T1715.

See: <https://github.com/facebook/phabricator/issues/461>

Test Plan: Downloaded a multi-hunk diff, got the original back and applied it cleanly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1715

Differential Revision: https://secure.phabricator.com/D7694
Fix two issues with shell/config scripts for hosted repositories

Summary: Ref T4151. `-ne` is numeric in some/most/all shells; `exec --` apparently doens't always work.

Test Plan: Will make @zeeg test.

Reviewers: btrahan, zeeg

Reviewed By: zeeg

CC: zeeg, aran

Maniphest Tasks: T4151

Differential Revision: https://secure.phabricator.com/D7702