LU-3289 gss: Add two additional security flavors for sk

LU-3289 gss: Add two additional security flavors for sk

This patch adds shared key null (skn) and shared key auth (ska)
flavors to make shared key consistent with the kerberos
implementation.  Shared key null requires a key to establish the
security context but does not use integrity or privacy outside of
the SEC_CTX_INIT RPC.  Shared key auth enables integrity for normal
service but not bulk.

Signed-off-by: Jeremy Filizetti <jeremy.filizetti@gmail.com>
Change-Id: I55fa52dfe1089f3dc9a40ffad28997a0b08aadec
Reviewed-on: http://review.whamcloud.com/18773
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>