TESTS: Add integration tests for the sssd-secrets
    
    Implements a simple HTTP client and uses it to talk to the sssd-secrets
    responder. Only the local provider is tested at the moment.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3054
    
    Reviewed-by: Petr Čech <pcech@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>