Add clarifications to manualtest-guide.md
Add clarifications to manualtest-guide.md

* Clarified how to set up for Kerberos on Linux
* Removed manual guide for Https/ClientCredentialTypeTests.cs

shmao/wcf,ElJerry/wcf,hongdai/wcf,ElJerry/wcf,iamjasonp/wcf,MattGal/wcf,iamjasonp/wcf,KKhurin/wcf,zhenlan/wcf,StephenBonikowsky/wcf,StephenBonikowsky/wcf,mconnew/wcf,khdang/wcf,shmao/wcf,ericstj/wcf,KKhurin/wcf,hongdai/wcf,dotnet/wcf,mconnew/wcf,khdang/wcf,zhenlan/wcf,MattGal/wcf,imcarolwang/wcf,ericstj/wcf,imcarolwang/wcf,dotnet/wcf,mconnew/wcf,imcarolwang/wcf,dotnet/wcf