Add git clone --recurse-submodules

[aknysh]

what

• Add git clone --recurse-submodules

why

• To recursively clone all git submodules into the directory
• addresses Support Git Submodules #4

test

=== RUN   TestCreateGitCloneCommand
Found Git version: 2.17.1
git clone arguments: [git clone --recurse-submodules https://github.com/runatlantis/atlantis /tmp/runatlantis/atlantis]
--- PASS: TestCreateGitCloneCommand (0.02s)

[sdwerwed]

@aknysh the submodule is empty, this doesn't clone the module for other branches, you need to explicitly specify -b <branch_name>, otherwise the module will be empty once do checkout. You can replicate by creating a new branch and add the submodule, then do PR to master.