Add flexibility to system packages that are controller by the Gitlab mod...

...ule

Currently, the list of `$system_packages` in Gitlab module, might interfer
with the setup one has already in place (ie. Duplicate declaration error).

In order to still provide a functionally working installation by default,
but yet avoid Duplicate declaration error on specific this commits allows
one to specify which `$system_packages` should be controlled by the Gitlab module.

So if one has the puppetlabs/gcc module enabled on the system, one could specify
the following `$system_packages` parameter :

```
$system_packages = ['libicu-devel', 'perl-Time-HiRes','libxml2-devel',
                    'libxslt-devel','python-devel','libcurl-devel',
                    'readline-devel','openssl-devel','zlib-devel',
                    'libyaml-devel','patch']
```

Note how gcc-c++ is not specified.

PS. I know that this module is using ensure_packages(), to check if a package is present, but another module might not, hence resulting in the error mentioned