It is simply the best/simplest solution and trying to prevent users from passing and integer here would be like opening a can of worms. Because:
* we cannot make mapstructure validate our duration string ( with an UnmarshalJSON func etc.)
* we cannot make mapstructure spit a string instead of a duration and packer will decode-encode-decode config.
* the hcl2 generated code asks for a string, so this will be enforced by default.
Before this commit it was possible to set a duration using an integer or a float. Go's time.Duration is an int64 internally an mapstructure will take advantage of this and load the number as a int64 but `1` means one ns which is unexpected/confusing. To avoid confusion and enforce readability this forces users to pass a string with a unit for a duration; ex "56s".
* add missing `temporary_key_pair_name` field for alicloud
* add missing description to `vpc_filter` for aws
* add missing ssh communicator fields for aws
* add links for aws
* update vmware-vmx docs
* manually describe AMI Configuration section for ebsvolume
* display missing required ami_name field for aws
* add missing fields for docker
* add missing fields for openstack
