Add support to use existing subnet to create the bastion instance.

[shb-mll]

the module doesn't have the provision to take in an existing subnet as the required subnet to bring up the bastion instance in. It always creates a new subnet.
Also the module does not take in nsg id as a parameter to pass to the bastion compute instance.

we have a use case where we have a VCN with a bastion (public) subnet. the bastion instance should be placed in the existing bastion subnet to support OKE cluster.
So we can provide VCN id(exists in the module), subnet id(need this) and NSG id(need this) as inputs to create the bastion instance.