Add `slave_alloc_update` tool.

Add `slave_alloc_update` tool.

Add the `slave_alloc_update` tool, which regenerates slave allocations
for waterfalls that are managed by the `common.slave_alloc`
SlaveAllocator.

In addition to the tool, this makes some supportive tweaks:
- Update master directory identification logic in `chromium_utils` to
  use global environment `build`/`build_internal` directories instead of
  generating them itself.
- Update master directory identification to handle master names that
  don't begin with `master.`.
- Update `slave_alloc` to store state options in the allocator, rather
  than have them be passed in when the state is saved. This enables
  consistent state saving regardless of whether the waterfall or
  `slave_alloc_update` is operating.

BUG=chromium:543290
TEST=local
  - Ran script myself, seems to work!

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=297262

[dnj, 2015-10-15 17:21:30]

Patchset #2 (id:20001) has been deleted

[dnj, 2015-10-15 18:46:54]

dnj@chromium.org changed reviewers:
+ stip@chromium.org

[dnj, 2015-10-15 18:46:54]

PTAL

This depends on:
https://codereview.chromium.org/1402253002

This has an internal component patch:
https://chromereviews.googleplex.com/270347013/

This is the second step in the chain, followed by:
https://codereview.chromium.org/1403313002

[ghost stip (do not use), 2015-10-21 16:26:06]

lgtm

https://chromiumcodereview.appspot.com/1393893004/diff/1/scripts/common/chrom...
File scripts/common/chromium_utils.py (right):

https://chromiumcodereview.appspot.com/1393893004/diff/1/scripts/common/chrom...
scripts/common/chromium_utils.py:1263: if not mastername.startswith('master.'):
weird. that should be for a user-facing util, not down in the bowels of
chromium_utils

[dnj (Google), 2015-10-22 23:48:01]

The CQ bit was checked by dnj@google.com

[commit-bot: I haz the power, 2015-10-22 23:48:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1393893004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1393893004/1

[commit-bot: I haz the power, 2015-10-22 23:51:01]

Committed patchset #1 (id:1) as
http://src.chromium.org/viewvc/chrome?view=rev&revision=297262