Multiple Tango Database support

Support Tango Multiple DBs URIs. To use it simply set the TANGO_HOST to TANGO_HOST=host1:port1,host2:port2,...hostN:portN. The database that will be used is resolved by Tango itself, which basically will get the first available database from the list. The hostN can be the alias, it's not required to use the full name.

Note: remember that it uses the first available database. If the GUI contains a device or attribute that exists in database2 (host2:port2) but doesn't exist in database1 (host1:port1), the GUI will show it as if the attribute doesn't exist if the database1 is available (because, indeed, it doesn't exist in the first available database).