Quite a few images are only used a few times in a few tests. Thus,
the images are being centralized into the agnhost image, reducing
the number of images that have to be pulled and used.
This PR replaces the usage of the following images with agnhost:
- net
- netexec
- nettest
- webhook
For windows node, security context is disabled. This PR fixes a bug so
that fsGroup will not be applied to pods that run on windows node.
Change-Id: Id9870416d2ad8ef791b3b4896d6747a2adbada2f