Added advanced input field Packer version, used to build the image.

[anuragc617]

This input will give an option to user to provide the packer version to use to build the image. With this input now the packer build task will behave as follows:

1. If packer version is provided then the highest of installed packer version on the machine and provided will be used to build the image.
2. If the packer version input is not provided then the highest of installed and maximum supported version will be used to build the image.

[bishal-pdMSFT]

@anuragc617 @sachinma if we want the task to use user provided Packer version, should we rather have a separate tool installer task on the lines of "Use Packer"?
This will ensure that the task is supported only for a particular version of Packer and we do not get unnecessary support calls if user is trying different packer version.

[sachinma]

@bishal-pdMSFT is this different from when the user has Packer version already present on the machine. Should we try to mitigate the support risk that you see by adding appropriate help text?

[bishal-pdMSFT]

@sachinma the machine packer version support is more of a "backdoor" to be used in a minimal set of cases. Adding an explicit version input will make it appear that the task is designed to work with any packer version.
I know that there is a very thin line between these two, but other thought was to align with how other tools are installed and consumed - .net core etc.