fix(scan): skip wordpress scan for preudo servers

[kotakanbe]

What did you implement:

Scan with the following config.toml, vuls connects via ssh and issues wp command, which results in an error.

[default]
  [default.wordpress]
    osUser = "hoge"
    docRoot = "/path"
    cmdPath = "fuga"

[servers]
[servers.localgem]
type = "pseudo"
lockfiles = [
  "/home/ubuntu/work/redmine/gem1/Gemfile.lock"
]

[Jan 21 06:28:24] DEBUG [localhost] execResult: servername: localgem
  cmd: /usr/bin/ssh -tt -o StrictHostKeyChecking=yes -o LogLevel=quiet -o ConnectionAttempts=3 -o ConnectTimeout=10 -o ControlMaster=auto -o ControlPath=/home/ubuntu/.vuls/controlmaster-%r-localgem.%p -o Controlpersist=10m @ -p  stty cols 1000; sudo -u hoge -i -- fuga cli version --allow-root
  exitstatus: 255
  stdout: 
  stderr: usage: ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
           [-D [bind_address:]port] [-E log_file] [-e escape_char]
           [-F configfile] [-I pkcs11] [-i identity_file]
           [-J [user@]host[:port]] [-L address] [-l login_name] [-m mac_spec]
           [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address]
           [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]]
           [user@]hostname [command]
  err: %!s(<nil>)
[Jan 21 06:28:24] ERROR [localhost] Error on localgem, err: [Failed to scan WordPress:
    github.com/future-architect/vuls/scan.GetScanResults.func1
        /home/ubuntu/go/src/github.com/future-architect/vuls/scan/serverapi.go:646
  - Failed to exec `sudo -u hoge -i -- fuga cli version --allow-root`. Check the OS user, command path of wp-cli, DocRoot and permission: &config.WordPressConf{OSUser:"hoge", DocRoot:"/path", CmdPath:"fuga"}:
    github.com/future-architect/vuls/scan.(*base).scanWordPress
        /home/ubuntu/go/src/github.com/future-architect/vuls/scan/base.go:633]
Scan Summary
================
localgem        Error           Use configtest subcommand or scan with --debug to view the details

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

scan works fine with the above config.toml

Checklist:

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES