Add a utility function to try an RPC until it returns True
Add a utility function to try an RPC until it returns True

Due to the still existing configuration lock, modifications
to the configuration can temporarily be impossible. Therefore,
most configuration-modifying function return a Boolean indicating
whether the change was carried out. Add a utility function to
retry that change until it succeeds.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>

leshchevds/ganeti,ganeti/ganeti,andir/ganeti,leshchevds/ganeti,mbakke/ganeti,bitemyapp/ganeti,andir/ganeti,mbakke/ganeti,mbakke/ganeti,grnet/snf-ganeti,onponomarev/ganeti,grnet/snf-ganeti,leshchevds/ganeti,ganeti/ganeti,bitemyapp/ganeti,onponomarev/ganeti,andir/ganeti,ganeti/ganeti