cmake: bump target jdk to 1.7

cmake: bump target jdk to 1.7

jdk 1.7 was released on 2011. For CentOS 6/7, both have openjdk
1.8. For Debian 8(jessie), Ubuntu 14.04(trusty) have openjdk 1.7.
These are the oldest distributions officially built for during
luminous release.

Since jdk 1.9/9, the minimal supported version is 1.6/6.
jdk 1.0/10(recently released) only supports 1.7/7.
http://openjdk.java.net/jeps/182

Fixes: http://tracker.ceph.com/issues/23458
Signed-off-by: Shengjing Zhu <zhsj@umcloud.com>