Remove hard dependency from tilestream

[daniviga]

We discovered that the platform has an hard dependency on tilestream which is a very fragile service (unmaintained software on an unsupported os, known to crash after X days).

We should, if possible, remove the tight dependency on Tilestream: if it is not available we should just be unable to load its layers, it should not prevent the platform and other maps to work if those layers aren't required/loaded.

The applications affected : Map's visualization, GeoExplorer, Grv