ceph.spec.in: clarify two important comments

ceph.spec.in: clarify two important comments

First, the terms "common" and "specific" are vague. Second,
"common" can easily be confused with the ceph-common subpackage.

Fix this by rephrasing to "distro-unconditional dependencies" and
"distro-conditional dependencies", respectively.

Third, move the "distro-unconditional dependencies" header so it
is above the part where these dependencies are actually defined.

Signed-off-by: Nathan Cutler <ncutler@suse.com>