Server adapter wizard: Improve source path validation

First point: In New OpenShift 3 server adapter wizard there is a text label for Source path. Currently there is only not null validation, if there is no path, next button is disabled. Because the path filled in this text widget is local, it would be nice if there would be simple validation of existence of directory described in this text widget.

Second point: validation message shown in header for empty source path is same as for the pod path "Please provide a path to deploy to on the pod.", what is not correct. This message should be different.

Third point: it would be nice to have an asterisks on required fields, such as pod or source path.