core: avoid saving duplicate records when saving LUN connections

When adding a LUN to the db, StorageHandlingCommandBase.proceedLUNInDb()
is executed. Among its other operations, this method checks if
connections with the same details as the LUN connections already exists
by executing Getstorage_server_connectionsByKey and if not a new connection
record is being saved, otherwise the existing one is used.

Getstorage_server_connectionsByKey currently checks if connection with the same
details exists according to the iqn, connection, port, portal username
and password fields - the problem is that those fields could be
set and persisted with with empty string or null, which will lead to
duplicate records.

*This patch only avoid having empty string in those fields - further
patch will fix the current state in the db and will add proper
contraints.

*Further fix that's done in this patch is to remove the password field
from Getstorage_server_connectionsByKey - each time we save/fetch a
connection from the db by it's details we encrypt the password - as the
encrypted password is different each time, we'll never be able to fetch
an existing record.

Change-Id: I997ba0fbc6e984a9e6782502c6b36b1f9a1f3d8c
Bug-Url: https://bugzilla.redhat.com/1173951
Signed-off-by: Liron Aravot <laravot@redhat.com>