Changed credentials validation check command

Since SCVMM power operations support was implemented we are using Powershell to execute commands on the SCVMM box. The arbitrary command 'uname -a' was used to check if credentials are valid, however this command is not supported in a powershell session causing an error to be returned in the log files.
This PR updates that command to 'hostname' which is a command that can be run in Powershell
Merge pull request #295 from bronaghs/scvmm_credentials_validation_cmd

Changed credentials validation check command