Add option to use buildbot 0.8.4p1 for slaves.
Add option to use buildbot 0.8.4p1 for slaves.

This can be done by supplying --use_buildbot_8 to run_slave.py. Eventually it
should be determined automatically from the master or whatever necessary.

BUG=
TEST=


Review URL: http://codereview.chromium.org/8686001

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@113829 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build