* Change generated documentation to indicate what image selection
options are required, and when.
* Refactor the structure of the documentation to highlight required
options for different types of builds
Updates azure builder to support setting end-of-life-date for
image versions in shared image gallery. In addition, this change adds
the ability to set the global replica count for the image version
and allows the vms deployed from the latest version of the Image
Definition to exclude this Image Version.
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".
