do_cmake.sh: Add CEPH_GIT_DIR

do_cmake.sh: Add CEPH_GIT_DIR

This patch adds a new env variables that users can set to define the
location of the checked out ceph git repo.

The documentation mentions having to go edit the script directly to
point to the ceph git directory if the build dir isn't in the
top directory of the checked out git repo. This patch changes the '..'
into an env variable, CEPH_GIT_DIR, that defaults to '..'. To make it
easy to script and change.

This allows someone to do things like:

 export CEPH_GIT_DIR=~/git/ceph
 ./do_cmake.sh

or

 CEPH_GIT_DIR=~/git/ceph ./do_cmake.sh

Which is much better then editing the script directly.
The README.md has been modified to mention this variable and also
includes a drive by to mention `make -j` for new contributors to
build ceph quicker.

Signed-off-by: Matthew Oliver <moliver@suse.com>