Moving caching code to Git.Clone method
Changes to clone fix per code review
fix dropbox functional test
Adding retry code to Git.Clone for parity with private
Several test updates:

1) Updated HttpClientHelper to expose statics for creating an HttpClient and an HttpClientHandler which is correctly set up to handle authentication. It uses a CredentialsCache along with preauthentication which handles authentication across redirects.

2) Updated KuduRemoteClientBase to make not force a teminating "/" on the base URI. This makes it possible to VFS to use the client base. Also cleaned up the class so that it is easier to use,

3) Added RemoteVfsManager to ApplicationManager which correct base URI and credentials.

4) Updated places which created an HttpClientHandler manually to instead use HttpClientHelper.CreateClientHandler
Improve repo cache logic to clean up the cache instead of cloning
Clean and use vfs helper

- Add simple helpers to RemoteVfsManager
- Use a separate RemoteVfsManager instance for wwwroot files
- Use it to create default.htm in 'reuse' mode
Make the repo cache logic more resilient
Use DeleteDirectorySafe helper
Delete unwanted default docs during site reuse
Default to site reuse mode
Support optionally deleting wwwroot when deleting the repo

Going through Vfs to clean the web root was proving difficult, and doing it as part of repo deletion is more reliable
Four updates:

1) Enabled support for wildcard etag. This allows the caller on PUT to use If-Match: * to match any exiting file.

2) Updated test to handle slash and turned on terminating slash for RemoteVfsManager

3) Added support for deleting empty directories.

4) Fixed bug in MediaTypeMap.GetMediaType not catching if we get a null registry key.
Pass EntityTagHeaderValue.Any instead of getting ETag via HEAD req
Move UrlUtility.EnsureTrailingSlash to KuduRemoteClientBase
Remove old file API
Unblock many folders & extensions that break vfs
support filter and diagnostic settings
Disable webpages to support cshtml files in vfs
Modifying VfsControllerBaseTest.cs to fix broken VFS tests.