Remove unused create_image of Nova v2 API

There are two create_image methods for Nova v2 API, but the one of
images_client.py only is used in most cases.
This patch removes the unused create_image of servers_client.py.

Change-Id: Icd457b6693c076c5c8b9d48bb88da162161c9a1a
Add opportunity to directly update headers

Added change in request's methods in rest_client
for directly updating headers.
If someone want to add some custom headers to default headers
he can pass variable extra_headers=True in one of
the request's methods(POST/PUT/...) and headers should be
added to default headers

Change-Id: Ifbe49a9f7a6e2de780953ef9dcd39e443d74d151
Closes-bug: 1295661
Updated from global requirements

Change-Id: I3bc2ae9ccf8f5cb6f590dc51de389bfd7e920aa3
Merge "Updated from global requirements"
Merge "Remove unused create_image of Nova v2 API"
Merge "Add opportunity to directly update headers"