Add warning before installation for advanced users

[ingenieroariel]

If a user already has old versions of many of the dependencies, hell can break loose. Let's add a warning for them in the installation page for both 1.2 and 2.0 as there may be potential for data loss during the confusion.

[ingenieroariel]

@capooti mentioned that for advanced developers a guide is preferred. A package is just looking for trouble. Let's write this guide and link to it there.

[capooti]

good point. I can volunteer on this, but will need a review as there is still something not clear to me.

[simod]

Any updates?

[ingenieroariel]

Let's add a warning here:
https://github.com/GeoNode/geonode/edit/master/docs/tutorials/admin/install/quick_install.txt
and here:
https://github.com/GeoNode/geonode/blob/master/docs/tutorials/admin/install/complete_install.txt
to point developers to the developer install section, if they have installed a lot of the pre-requisites already.

Marking as an easy-pick.