connectors: use ServiceLoader to select the default connector

This patch takes advantage of ServiceLoader to automatically select a
default connector (DEFAULT_CONNECTOR). It is still possible to specify
a custom connector at the moment of the creation of any OpenStackClient.

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Merge pull request #32 from simon3z/connectors

connectors: use ServiceLoader to select the default connector