Add REP 137 (rosdistro files)

This REP defines and specifies rosdistro files needed
to centralize ROS distribution information and to ease
building ROS on custom buildfarms
Took community comments into account
Merged file format & specification

Also changed the types to simpler words to avoid confusion
Clarify master file specification

 - doc folder
 - distribution cache
Added status to repositories in distribution file

Also gave reference API implementation
Fixed examples and test file spec
Added admin-notification to build file

also fixed example titles
Added references
Fixed RST reference formatting
Spellchecking
update packages element in distribution doc to be more specific.
update notification section
updating documenatation reference to be less specific explicitly.  and updating examples
dashes to underscores
Updated build file spec

Need reviewing, especially for the test build files
updated build file spec
Updated build file spec
Add optional tags dict to repositories in distro

This provides a useful future proofing
mechanism for tools like bloom and the
rosinstall generator which need to know
the format of tags created in release
repositories, which may not be uniform
over time.
Update rep-0137.rst