Document upgrade_tomcat.ps1 prerequisites and detailed how-to

When running the script upgrade_tomcat.ps1 documented at https://doc.nuxeo.com/810/nxdoc/hotfixes-installation-notes-for-nuxeo-platform-lts-2016/#hotfix-15 , some improvements to the documentation are suggested:

• Download the PowerShell script upgrade_tomcat7.ps1 => The link to upgrade_tomcat.ps1 is NOT a download link, it links to github source code. So users download a HTML page instead of the script itself.

• Some prerequisites necessary to run this script are not listed:

1. open PowerShell command-line environment and do the following in that environment
2. ensure java.exe can be run with the current $env:path environment variable or set the path as needed e.g. $env:path="c:\jdks\jdk1.8.0_141\bin;$env:path"
3. get the default permissions to run script
   Get-ExecutionPolicy -List
   (assuming here "Undefined" is returned for the scope "CurrentUser")
4. set the permissions to run this script:
   Set-ExecutionPolicy Unrestricted -Scope CurrentUser
5. run the script:
   upgrade_tomcat.ps1 NUXEO_HOME [Tomcat_version]
6. restore the default permissions to run scripts (assuming "Undefined" here)
   Set-ExecutionPolicy Undefined -Scope CurrentUser
7. exit PowerShell