Add new project properties to documentation

[salv-orlando]

Add description for:

• activate_default_dfw_rules
• external_ipv4_blocks
• tgw_external_connection

[vmwclabot]

@salv-orlando, you must sign every commit in this pull request acknowledging our Developer Certificate of Origin before your changes are merged. This can be done by adding Signed-off-by: John Doe <john.doe@email.org> to the last line of each Git commit message. The e-mail address used to sign must match the e-mail address of the Git author. Click here to view the Developer Certificate of Origin agreement.

• Commit a42b601d46c9af393f391c1b4a864ce82d627f39 must be signed by salv.orlando@gmail.com