Fixed URI credentials setup

Make sure the ?credentials=kiwiRepoCredentials flag is only added
to the url if the appropriate credentials file exists
Fixed passthrough of luks cipher (bnc #898249)

kiwi passed the ciper using a shell echo command which is
bad because the shell evaluates the contents of this information
and might break the cipher if it contains shell meta characters.
This patch fixes this by using a simple pipe stream to the
cryptsetup process without invoking a shell command
Fixed double encoded luks system (bnc #898249)

when kiwi builds a disk image it can either dump a filesystem
image onto a raw partition or it rsync a tree of files onto
a partition which has a filesystem created on top of a subsystem
or a subsystem stack like lvm, luks, etc. In kiwi this is
controlled by the value of the $treeAccess variable. If
treeAccess is disabled but the support for syncing a tree in
the given configuration is implemented this results in
duplicate preparation of the filesystem and the subsystems
which results in an unbootable mess.