Fix callhome device "DISCONNECTED" status

Fix callhome device "DISCONNECTED" status

Fixed error when callhome device always has status "DISCONNECTED"
after adding it to allowed devices. This issue arose after refactoring
AuthorizedKeysDecoder class to use plain KeyStore access to acquire
provides. New code did not specify what exact provider we want to use
and thus caused confusion - instead of Bouncy Castle KeyFactory that
was used before new code returned Sun KeyFactory. That result in next
issue with searching for valid host key when adding new device and
all other errors that described in logs attached to Jira ticket.

JIRA: NETCONF-1249
Change-Id: I5689fa6ad173a5b7cd99f0210a366b77db74b0d5
Signed-off-by: Oleksandr Zharov <oleksandr.zharov@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>