Set the cookie path to be SITE_ROOT.

The cookie path was being set to SITE_ROOT, but too soon, before
SITE_ROOT was set in settings_local. Now it's set after we've loaded
settingS_local.
Fix the PYTHONPATH issue for good (hopefully).

The latest change to set PYTHONPATH to be limited to the location of the
settings_local.py file broke virtualenvs, which no longer set PYTHONPATH
correctly and break rbssh.

PYTHONPATH seems to be a complicated beast when virtualenvs and
pkg_resources comes in, so we're just going to stop dealing with it
here. Instead, we're using a new variable, RBSITE_PYTHONPATH, which will
point to any paths we have within a site that we care about. For now,
this is the conf/ directory. rbssh will specifically look for this and
add the values to sys.path.
Allow specifying the MySQL port in rb-site.

When configuring a site using 'rb-site install', the hostname of a MySQL
database can be specified in hostname:port format. This works for the
wizard and for the --db-port option.

Reviewed at http://reviews.reviewboard.org/r/3923/
Release Review Board 1.7.7.
Merge branch 'release-1.7.x'