Initial support for the `ext' template in gnt-instance modify.
Take care of all needed checks in CheckArguments and CheckPrereqs
wrt ExtStorage Providers and ext-params.

ExtStorage disk addition and specification of arbitrary ext-params
during addition is supported.

Currently, one cannot modify an existing disk's ExtStorage Provider,
neither can change an existing disk's ext-params. Only an existing
disk's `mode' can be modified.

Removal is transparent of the disk template and is also supported.

Signed-off-by: Constantinos Venetsanopoulos <cven@grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
[iustin@google.com: fix bug in disk changes, pylint fixes]
Reviewed-by: Iustin Pop <iustin@google.com>