Clean up SessionService POST

Previously, SessionService would return a json decode error for all
errors, even if certain keys were incorrect.  This updates the POST
handler to use a better method, and should reduce the binary size a
little, given that we're removing the possibly throwing get<> code.

This needs a very good review.  This endpoint is available pre-auth, and
needs to be secure regardless of input.

Tested By:
curl --noproxy "*" -k -X POST -d '{"UserName": "root", "Password":
"0penBmc"}' https://10.243.48.30/redfish/v1/SessionService/Sessions/
Returns correct response.
Tested several variations, including extra parameters, missing username,
missing password, ect.  Got correct error code in all cases.

Change-Id: Ic8e6edd9f7badc0d22aa8dafa9c0c260386712ac
Signed-off-by: Ed Tanous <ed.tanous@intel.com>

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