Set the kadeploy VM mem to 4G, to fix pb with the mysql2 gem installation

Reported error:
    kadeploy: + gem install --conservative --no-ri --no-rdoc mysql2
    kadeploy: Building native extensions. This could take a while...
    kadeploy: ERROR:  Error installing mysql2:
    kadeploy:     ERROR: Failed to build gem native extension.
    kadeploy:
kadeploy:     current directory:
/var/lib/gems/2.5.0/gems/mysql2-0.5.5/ext/mysql2
kadeploy: /usr/bin/ruby2.5 -r ./siteconf20230615-12767-qbk791.rb
extconf.rb
kadeploy: Cannot allocate memory - /usr/bin/ruby2.5 -r
./siteconf20230615-12767-qbk791.rb extconf.rb 2>&1
    kadeploy:
kadeploy: Gem files will remain installed in
/var/lib/gems/2.5.0/gems/mysql2-0.5.5 for inspection.
kadeploy: Results logged to
/var/lib/gems/2.5.0/extensions/x86_64-linux/2.5.0/mysql2-0.5.5/gem_make.out
The SSH command responded with a non-zero exit status. Vagrant
assumes that this means the command failed. The output for this command
should be in the log above. Please read the output to determine what
went wrong.