Make git scm easier configurable (e.g. for forks / sandbox usage) (#541)

* Add hard-coded pom version, otherwise release will fail with error

"There are no snapshot projects that could be released!" due to auto-updated parent version
* Make git scm easier configurable (e.g. for forks / sandbox usage)

Signed-off-by: Patrick Fink <mail@pfink.de>

pom.xml

@@ -26,8 +26,8 @@
     </licenses>
 
     <scm>
-        <connection>scm:git:https://github.com/openhab/openhab-distro.git</connection>
-        <developerConnection>scm:git:https://github.com/openhab/openhab-distro.git</developerConnection>
+        <connection>scm:git:${scm.gitBaseUrl}/openhab-distro.git</connection>
+        <developerConnection>scm:git:${scm.gitBaseUrl}/openhab-distro.git</developerConnection>
         <url>https://github.com/openhab/openhab-distro</url>
     </scm>