Unify multi-instance handling in gnt-instance
Unify multi-instance handling in gnt-instance

Start, stop and reboot have the same code for dealing with
multi-instance handling. This patch moves all that into a single generic
function, and leaves only the building of the specific opcode for the
operation in the individual functions.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>

bitemyapp/ganeti,onponomarev/ganeti,grnet/snf-ganeti,apyrgio/snf-ganeti,ganeti/ganeti,leshchevds/ganeti,apyrgio/snf-ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,dimara/ganeti,ganeti-github-testing/ganeti-test-1,grnet/snf-ganeti,yiannist/ganeti,onponomarev/ganeti,apyrgio/ganeti,mbakke/ganeti,ganeti/ganeti,dimara/ganeti,yiannist/ganeti,andir/ganeti,mbakke/ganeti,andir/ganeti,ganeti/ganeti,bitemyapp/ganeti,yiannist/ganeti,apyrgio/ganeti,leshchevds/ganeti,andir/ganeti