src/common/version: append vendor release to ceph version(s) output

src/common/version: append vendor release to ceph version(s) output

Introduce a build-time mechanism to allow vendors or downstream
distributions to append their release/version metadata to the output of
ceph version and ceph versions.

The release string is read from the file `/etc/ceph_version` and appends
it to the version string as `release <value>`. If no value is supplied
or the file is empty, the version output remains unchanged.

Example:
If the `/etc/ceph_version` file has "8.1.0" as string then,
ceph version 8d7daada90 (98d7daada90c6585c6724f69793e804a5e23cb24) tentacle (dev - Debug) release 8.1.0

Signed-off-by: Naveen Naidu <naveen.naidu@ibm.com>