Fixes to get builds and tests running on RHEL:

  Added --base-os option which takes "fedora" or "rhel" to inidicat which OS is being used on ec2
    - This defaults to the same os as the build/launch/test is invoked from
    - ignored packages, ssh user, ami images etc are loaded based on this option
    - Adds EPEL as a repo on RHEL Origin builds
  Removed duplicate definition of sanity_check function
  Fixed dependencies installed on RHEL vs Fedora in the install-required-packages task
  Use -t when making ssh commands to open pseudo tty during test runs (needed by rhc spec tests)
  Update tests runner to use scl ruby193 on RHEL Origin tests only
  Fixed which services are restarted after launching an Origin image
  Removed dependence on redhat-lsb to detect operating system
  Made initial gem install scl aware for RHEL