Ability to specify Jib version.

[saturnism]

Currently, Jib builder always use the latest version of the released plugin. For consistency/reproducibility, user may want to specify a specific version of Jib.

Add JIB_VERSION argument and default to RELEASE.
In the Jib step, use plugin coordinate com.google.cloud.tools:jib-maven-plugin:${JIB_VERSON}:build

[saturnism]

/cc @coollog wdyt?

[coollog]

Sounds good! Will add this in

[saturnism]

hmm, i wonder if the version here will override the actual plugin version specified by the pom?
another way of doing this to specify the plugin version in the build file. that's actually really good too.

[coollog]

Yea, the user can definitely specify the plugin version in the build file, but the JIB_VERSION build template argument can be made to override that (so a default empty JIB_VERSION would call com.google.cloud.tools:jib-maven-plugin::build, which would use the project-specified version).