Field `owners` is required for `aws_ami`, otherwise it fails

Signed-off-by: Denis Iskandarov <d.iskandarov@gmail.com>

Signed-off-by: Denis Iskandarov <d.iskandarov@gmail.com>

* ability to append numerical suffix even to 1 instance
Signed-off-by: Denis Iskandarov <d.iskandarov@gmail.com>

* include use_num_suffix in README.md

* Allow specifying a list of subnet IDs to iterate instances over

* Include the module version and some code formatting

* Explicitly specify the version for SG AWS module

Any change in the ebs block device currently triggers re-creation of
this resource which I assume is not intended.

Workaround by ignoring these changes as suggested in:

https://github.com/terraform-providers/terraform-provider-aws/issues/1297#issuecomment-319359965

This narrows the match for things considered to be t2 instances to only
those types that start with "t2.". Previously there was a class of
(invalid) instance types, such as "2b.something" or "t5.something" that
would cause an error on boolean parsing by matching one regex and not
the other.

* Added support for CPU credits

* Updated formatting

Modify tag name management to add -%d suffixe only if instance_count > 1

Stop ignoring changes in vpc_security_group_ids

Renamed count to instance_count (fixes #23)