Launch several virtual machines simultaneously

Hi, @olivergondza

We encountered the following problem while using your plugin: new virtual machines are launched one by one, even when build queue is full.

Steps to reproduce:

- Two templates of slaves: CENTOS & REDHAT
- Settings are set in "Default slave options"
- Max. No. of Instances = 50
- Min. No. of Instances = 3
- Launch 15 long-running jobs

Observed behaviour:
- 6 job start on existing agents
- 9 jobs are hanging in the queue
- new agents are launched one-by-one, new vm is created only after a previous one is complete

What we want:
- some parameter, which allows us to set up maximum number of agents to be launched simultaneously
or
- the way to configure such behaviour by some combination of existing parameters

Below are the logs showing that creation of 3 instances is planned, but they are launched one-by-one.

> 2020-02-18 **08:26:51**.501+0000 [id=11476] INFO j.p.o.c.JCloudsPreCreationThread#execute: Pre-creating 3 instance(s) for template rhel7 in cloud cloud.xxx.com
>
> 2020-02-18 08:26:51.513+0000 [id=11476] INFO j.p.o.c.JCloudsSlaveTemplate#provisionServer: Provisioning new openstack server rhel7-162 with options jenkins.plugins.openstack.compute.SlaveOptions@517f51dd[...]
>
> 2020-02-18 **08:27:21**.872+0000 [id=11476] INFO j.p.o.compute.JCloudsSlave#createComputer: Creating a new computer for rhel7-162
>
> 2020-02-18 08:27:21.886+0000 [id=11476] INFO j.p.o.c.JCloudsSlaveTemplate#provisionServer: Provisioning new openstack server rhel7-163 with options jenkins.plugins.openstack.compute.SlaveOptions@5723a58f[...]
>
> 2020-02-18 **08:27:54**.541+0000 [id=11476] INFO j.p.o.compute.JCloudsSlave#createComputer: Creating a new computer for rhel7-163
>
> 2020-02-18 08:27:54.561+0000 [id=11476] INFO j.p.o.c.JCloudsSlaveTemplate#provisionServer: Provisioning new openstack server rhel7-164 with options jenkins.plugins.openstack.compute.SlaveOptions@457210bd[...]
>

Thanks in advance.