[loader] using provider loaders

Provider loader configuration accessing/registration, provider is loaded
using loader, depending on command-line option --statefs-type etc.

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[tests] updating tests to test/use loaders

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>