(maint) Allow git URI to be specified in local_options.rb

Previously, the Rakefile assumed if you were doing a git type
acceptance test that the repos to install were of the form
puppet#stable or just puppet. In either case, environment variables
GIT_SERVER, FORK, and SHA were used to generate the actual URI, e.g.
git://<server>/<fork>#<sha>

However, if the repo to install was an actual URI, then the Rakefile
would silently map each repo to nil, and would trigger the error
message added in 099b0cbd6.

This scenario would always happen if you ran the tests once, and copied
merged_options.rb to local_options.rb, and attempted to re-run the
tests with the same configuration as last time. In that case,
local_options.rb would contain URIs of the form:

:install=>
  ["git://github.com/puppetlabs/facter.git#stable",
   "git://github.com/puppetlabs/hiera.git#stable",
   "git://github.com/puppetlabs/puppet.git#master"
  ],

This commit modifies the Rakefile to preserve the specified URI if it
doesn't match one of our "short" versions, e.g. puppet#stable. This only
applies to git type tests.