Store "tag" and "url" in files in reservation dir

String "Unknown" is used if test run tag or url is not supplied,
so this change should be backward compatible.

+ Used "runtag", as both "test" and "tag" start with taken "-t".
+ Fixed some pylint violations.
+ Docstring explaining the steps taken and reasoning behind two files.
+ Other minor improvements.

Change-Id: If704d02e51374087fe39ad0f69432477d1b310d3
Signed-off-by: Vratko Polak <vrpolak@cisco.com>