Gerrit_plugins: Add support for VERSION file

Currently plugin version is always retrieved from underlying Git
repository with git describe command. Add support for VERSION file
in case the Git repository doesn't exist yet, or plugin authors
prefer to expose their plugin version through VERSION file (as
Gerrit itself).

Format of VERSION file is the same as for Gerrit core: it must be
valid python file, that is sourced by BUCK and include

  PLUGIN_VERSION = 42

Change-Id: Iba511a9fcf16b4324d7ba2cd4c131d89b564582c
1 file changed