Restore SMM web request semantics lost in the async port

The old WebConnection.request() added a
'Content-Type: application/x-www-form-urlencoded' header to any POST
carrying a body, but grab_response_with_status() only sets a content
type for dict payloads, so the SMM login and every /data form POST now
go out as text/plain.  This is not a fix for an observed failure: an SMM
running FPC variant 38 was measured accepting a text/plain login exactly
as readily as a urlencoded one.  It restores the header the synchronous
code always sent and that the TSM and IMM handlers still set explicitly,
rather than relying on every SMM firmware level being equally lax about
what it will parse.

Also stop hard failing on responses the synchronous code discarded on
purpose.  'set=securityrollback:1' is only understood by newer SMM2
firmware.  And /data/logout answers 401 once the session is gone, as
measured on that same SMM, so raising on a non-200 there turns a
completed hostname, domain or NTP operation into a spurious error.

confluent_server/aiohmi/ipmi/oem/lenovo/nextscale.py

@@ -736,11 +736,9 @@ class SMMClient(object):
             if not isinstance(username, str):
                 username = username.decode('utf8')
             wc = await self.wc()
-            rsp, status, _ = await wc.grab_response_with_status(
+            await wc.grab_response_with_status(
                 '/data', 'set=user({0},1,{1},511,,4,15,0)'.format(
                     uid, username))
-            if status != 200:
-                raise Exception(rsp)
 
     async def reseat_bay(self, bay):
         bay = int(bay)
@@ -831,6 +829,7 @@ class SMMClient(object):
     async def get_webclient(self):
         cv = self.ipmicmd.certverify
         wc = webclient.WebConnection(self.smm, 443, verifycallback=cv)
+        wc.set_header('Content-Type', 'application/x-www-form-urlencoded')
         wc.vintage = util._monotonic_time()
         loginform = urlencode(
             {
@@ -1016,9 +1015,8 @@ class SMMClient(object):
         if status != 200:
             raise Exception('Error validating firmware')
         progress({'phase': 'apply', 'progress': 0.0})
-        rsp, status, _ = await wc.grab_response_with_status('/data', 'set=securityrollback:1')
-        if status != 200:
-            raise Exception(rsp)
+        # only understood by newer SMM2 firmware, ignore rejection by older
+        await wc.grab_response_with_status('/data', 'set=securityrollback:1')
         rsp, status, _ = await wc.grab_response_with_status('/data', 'set=fwUpdate:1')
         if status != 200:
             raise Exception(rsp)
@@ -1094,9 +1092,8 @@ class SMMClient(object):
         self._wc = None
         if wc is None:
             return
-        rsp, status, _ = await wc.grab_response_with_status('/data/logout', None, method='POST')
-        if status != 200:
-            raise Exception(rsp)
+        # best effort, a stale session must not fail the caller's operation
+        await wc.grab_response_with_status('/data/logout', None, method='POST')
 
     async def wc(self):
         if (not self._wc or (self._wc.vintage