Make the OPENVAS_DEFAULT_SOCKET configurable via cmake.

[cfi-gb]

This is a follow-up of #1238 to make the default path configurable on build time. It will be also used for #1240 to set the OSP_VT_UPDATE in the gvmd.default in a second step (and once #1240 was merged).

To test i have called cmake with an additional -DOPENVAS_DEFAULT_SOCKET=/test/ospd/ospd.sock, after setting up a fresh database i have verified that the default socket is directly configured in the database:

$ gvmd --get-scanners
08b69003-5fc2-4037-a479-93b440211c73  OpenVAS  /test/ospd/ospd.sock  0  OpenVAS Default
6acd0832-df90-11e4-b9d5-28d24461215b  CVE    0  CVE

The same was also done without the cmake config where the scanner is having the default path:

$ gvmd --get-scanners
08b69003-5fc2-4037-a479-93b440211c73  OpenVAS  /var/run/ospd/ospd.sock  0  OpenVAS Default
6acd0832-df90-11e4-b9d5-28d24461215b  CVE    0  CVE

Checklist:

• Tests
• CHANGELOG Entry