Handle case when virtualenv_version fact is not available.

As specified in #94, when their is no way to find virtualenv version,
fall back to user request.

When fact virtualenv_version is not yet available, stupidly trust user
input and force systempkgs options without taking care of virtualenv
veresion.

1. If the option choosen is not recognized by the installed version of
virtualenv, further Exec will fail and dependencies will not be processed.
2. On the next puppet execution, fact _virtualenv_version_ will be available and process should work as
expected.
Merge pull request #95 from ccin2p3/systempkg_when_no_fact_available

Handle case when virtualenv_version fact is not available.