Allow midostack script run from packages

Four new environment variables have been added:

* MIDOSTACK_MIDOLMAN_PACKAGE_URL
* MIDOSTACK_MIDONETAPI_PACKAGE_URL
* MIDOSTACK_NEUTRONPLUGIN_PACKAGE_URL
* MIDOSTACK_MIDONETCLI_PACKAGE_URL

If first two are informed (they must be both because they belong to the
same GitHub repository!) with the URL of these packages, Midostack will
download them and install them via 'dpkg -i' instead of compile the
packages.

If last two are informed (they must be both because neutronplugin
depends on the midonetcli and midonetcli has to be installed
previously), midostack will download the package and install it instead
of clone the respective repositories.