Bug Fixes and Improvements to 2.0

Major Changes:

* Convert rez-cmake to new-style subcommand of rez entry point.  Create new cmake.py module to collection functionality shared by rez.cli.cmake, rez.cli.build and rez.release.
* Strip out rez-build after the exit call to remove redundant code.
* Change the order the environment variables in init.csh|sh are set such that the local and release packages env. vars. can be reused instead of being hardcoded.
* Derive the packages search path from the local and release packages env. vars. in init.csh|sh instead of hardcoded strings (substitued at configure time) for more flexible behaviour/usage.

Minor Changes:

* Set a package_ROOT CMake variable in rez_install_cmake.
* Print a footer message on variant build.
* Make the VCS tag name configurable.
* Make the default CMake build system and arguments configurable.

Patches:

* Fix developer warning in Utils.cmake.
* Fix a bug to preserve backwards compatibility with CMAKE_MODULE_PATH handling.
* Fix a bug in kwargs passed to ActionManager