Update rpm build scripts and README

Update rpm build scripts and README

Remove the unused Dockerfiles.

Create two flavors of spec files, one which includes steps to build
clang from source and link against it, and one which assumes clang is
already available and just builds bcc. The former is more appropriate
for official packaging, the latter for nightly and quick builds.

Also, introduce a numbering scheme for nightly rpm files. The format
will be:
`libbcc-$latest_git_tag-g$latest_git_hash.x86_64.rpm`
as in:
`libbcc-0.1.7-g73c4d1a.x86_64.rpm`

Update the INSTALL.md to mention the RPMs

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>