Improve help strings

Improve consistency of help strings: Add missing ".", fix wording.

Change-Id: If9dba459dcbbf695b9057e82525a83371ceebd8b
Correct Content-Type/Accept management in HTTPClient/SessionClient

Currently SessionClient.request never specifies Content-Type/Accept
headers in requests where HTTPClient.request always specifies json
Content-Type/Accept even with "--request-format xml" option ... requests
succeed because neutron deduces Content-Type/Accept from url suffix when
defined.

This change ensures HTTPClient/SessionClient define Content-Type/Accept
headers when needed and align them with --request-format option.

Closes-Bug: #1368335
Change-Id: I909e055e5e59fa5870a28ebb0f171812acca0745
Merge "Correct Content-Type/Accept management in HTTPClient/SessionClient"
Merge "Improve help strings"