'--advanced' in the CLI and REST API during volume create doesn't intuitively
and clearly indicate that it's for the volume options. A new user may read this
as creation of an "advanced volume" which isn't true.
The flags have now been prepended with 'allow' in the fields of request structs.
The API has been renamed as 'allow-<type>-options'.
Also, created 'VolOptionFlags' struct for flags of options and embeded it in
all request structs.
Signed-off-by: Prashanth Pai <ppai@redhat.com>