Remove second database instance from tests (#630)

[mliszcz]

The PR removes second database instance as proposed in #630. It impacts #608 test scenario.

Without #608 fix, with second database the failure was:

Tango exception
Severity = ERROR 
Error reason = DB_DeviceNotDefined
Desc : device test/debian8/10 not defined in the database !
Origin : DataBase::ImportDevice()

Without second database, the failure is:

25: Tango exception
25: Severity = ERROR
25: Error reason = API_TangoHostNotSet
25: Desc : TANGO_HOST env. variable not set, set it and retry (e.g. TANGO_HOST=<host>:<port>)
25: Origin : Database::Database

[bourtemb]

Looks good to me! Good initiative.
Thanks @mliszcz