Fixup release dry runs; use isolated plugin cache.

Review Request #2874 - Created Sept. 23, 2015 and submitted

John Sirois
2248, 2256
benjyw, peiyu
Previously release dry runs used the global plugin cache.  This was
problematic since pants versions are static in between releases, and so
installation and use of plugins in release dry run tests would use the
last released version of a plugin instead of the version just burned to
`dist/`.  Combined with API changes across plugin boundaries since the
last release, this could lead to spurious failures.

Fix this by adding an advanced `--plugin-cache-dir` option and use this
in the release dry run pointing to a new empty temporary directory.

 build-support/bin/              | 12 +++++++++---
 src/python/pants/bin/   |  3 ++-
 src/python/pants/option/ |  4 ++++
 3 files changed, 15 insertions(+), 4 deletions(-)

Before the fix, ./build-support/bin/ failed consistently
for me, after the fix, it works.

CI went green here:

Benjy Weinberger
John Sirois
John Sirois
John Sirois
Review request changed

Status: Closed (submitted)