Avoid duplicating PermitUserEnvironment

Do not add a new line if the option is already defined. This avoids
adding dupulicated lines and also make sure that the existing explicit
setting to disable PermitUserEnvironment is purged.

Closes #1953

Author: kajinamit

lib/beaker/host/unix/exec.rb

@@ -302,7 +302,7 @@ def ssh_permit_user_environment
     case self['platform']
     when /amazon|debian|ubuntu|archlinux|el-|centos|fedora|redhat|oracle|scientific|opensuse|sles|solaris/
       directory = tmpdir
-      exec(Beaker::Command.new("echo 'PermitUserEnvironment yes' | cat - /etc/ssh/sshd_config > #{directory}/sshd_config.permit"))
+      exec(Beaker::Command.new("sed -e 's/^PermitUserEnvironment .*/PermitUserEnvironment yes/' -e t -e '1s/^/PermitUserEnvironment yes\\n/' /etc/ssh/sshd_config > #{directory}/sshd_config.permit"))
       exec(Beaker::Command.new("mv #{directory}/sshd_config.permit /etc/ssh/sshd_config"))
       exec(Beaker::Command.new("echo '' >/etc/environment")) if self['platform'].include?('ubuntu-')
     when /(free|open)bsd/

spec/beaker/host/unix/exec_spec.rb

@@ -79,7 +79,7 @@ def to_s
     describe '#ssh_permit_user_environment' do
       context 'When called without error' do
         let(:directory) { '/directory' }
-        let(:ssh_command) { "echo 'PermitUserEnvironment yes' | cat - /etc/ssh/sshd_config > #{directory}/sshd_config.permit" }
+        let(:ssh_command) { "sed -e 's/^PermitUserEnvironment .*/PermitUserEnvironment yes/' -e t -e '1s/^/PermitUserEnvironment yes\\n/' /etc/ssh/sshd_config > #{directory}/sshd_config.premit" }
         let(:ssh_move) { "mv #{directory}/sshd_config.permit /etc/ssh/sshd_config" }
 
         platforms = PlatformHelpers::REDHATPLATFORMS + PlatformHelpers::DEBIANPLATFORMS