tools: Use $PATH to determine location for phantomjs

Another place we want to use $PATH to determine the location of
phantomjs, in case it's installed to a home directory.

Closes #1233
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
ws: Make cookie be a session cookie

Although it really doesn't matter that much since we clear cookie's
on the server side anyway, make our cookie a session cookie, so
that it never gets stored in the browser.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
ws: Give our cookie a simpler name: cockpit

When embedding, people will probably start using this cookie's name
explicitly, so keep it simple and straightforward.

Closes #1234
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>