Log debug message when KDFKey is missing

Undo recent change that raised an error when KDFKey was not yet stored
as this situation is expected until client has requested a first batch
of TCerts.
This change partially affects changes submitted in PR #2177
(#2177).

Why:
Fixes #2275
#2275

How it was tested:
Unit-test and behave test passed

Checklist:
X I have added a Signed-off-by.
X This change requires no new documentation.
X This change requires no new tests.
X I have run golint and have fixed valid warnings in code I have added
or modified.

Change-Id: Id67589d822198400319830b17067e6fcf5d8832e
Signed-off-by: Jeronimo Irazabal <jeronimo.irazabal@gmail.com>

core/crypto/client_tca.go

@@ -78,7 +78,7 @@ func (client *clientImpl) loadTCertOwnerKDFKey() error {
 	client.Debug("Loading TCertOwnerKDFKey...")
 
 	if !client.ks.isAliasSet(client.conf.getTCertOwnerKDFKeyFilename()) {
-		client.Error("Failed loading TCertOwnerKDFKey. Key is missing.")
+		client.Debug("TCertOwnerKDFKey is missing, maybe the client has not requested any TCerts from TCA yet")
 
 		return nil
 	}