Description of problem:
>>>
Remote execution job fails with "TypeError (no implicit conversion of Array into String" error after upgrade to 6.7 when Global Parameter has "remote_execution_ssh_keys" defined. 

Version-Release number of selected component (if applicable):
>>>
6.7

Steps to Reproduce:
>>>
1. Add "remote_execution_ssh_keys" in Global Parameters with type string and any key value  like below,
--
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDSsK0oExZLLLVLS2ZN+8qTl2ak+4tRGTP57mTtlGd1nNd6aTFwDakQ6chW18VcrpELhU0VYzUBYI4K2IRlYQUolW3qCLIeKAYJHDj81KYdSWeGx4PPPskmUc3BczN2nZOb3+PoOsV00lH1rfPIFuA6Oj4TUiyCpKZIUIL6/tKrQ/WxZ5FDbdQyHicXtOQy/rroyoM/DArJm77LTC9V8RCDmBVioK4bltdQXSA4SQkmkIAn9hafNwmIbFq/9JRdFXYzj5pqWikXYj45BLcELlOoBoZZx6N9UelwcE/fFL/dy8hiWYzOkMxshuCTuVdAJi/rPedZd1XGSD455uboPlRJ
--

2.  Try to fetch host paras from Satellite webui >> Hosts >> select any host >> click on "YAML" or use hammer command, 

# echo  'Host.find_by_name("vm253-90.example.com").params'  |  foreman-rake console


Actual results:
>>>
[root@sat67 ~]# echo  'Host.find_by_name("vm253-90.example.com").params'  |  foreman-rake console
Loading production environment (Rails 5.2.1)
Switch to inspect mode.
Host.find_by_name("vm253-90.example.com").params
Traceback (most recent call last):
        6: from lib/tasks/console.rake:5:in `block in <top (required)>'
        5: from (irb):1
        4: from app/models/concerns/host_params.rb:11:in `params'
        3: from app/models/concerns/host_params.rb:54:in `host_params'
        2: from foreman_remote_execution (2.0.10) app/models/concerns/foreman_remote_execution/host_extensions.rb:54:in `host_params_hash'
        1: from foreman_remote_execution (2.0.10) app/models/concerns/foreman_remote_execution/host_extensions.rb:54:in `+'
TypeError (no implicit conversion of Array into String)

Expected results:
>>>
[root@sat66 ~]# echo  'Host.find_by_name("vm253-90.example.com").params'  |  foreman-rake console
Loading production environment (Rails 5.2.1)
Switch to inspect mode.
Host.find_by_name("vm253-90.example.com").params
{"anan"=>true, "enable-epel"=>"false", "enable-puppet5"=>"true", "remote_execution_ssh_keys"=>["ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDhZJllZtrkxCLRuHTPQdRGktroyY7S3Vmk6/QHtZSHCvV/OrtVYyaUGVGxB5nD+uzBnz2IkYp49lnPwPYqGX5S/gzzCbM4EoyGvulGCVTAq+TwKJUQkck2QmpTnyKNiZCGQlIlBE8aGSEfAwvfyUEkng+Rjsw4u/UoNYJ5ijuVNjyh6G6rIilT0f3MdQ/DQTmMQVy1Aol5li8MCbu2r96rgff2BHM2IdV1Aqr5hTbNF/+SbhKZWq1nTZ4Jmbu9hSux/l4atxH7ikqVucP2HZEDtDY6zAvHSgMSJtEcdC38GZ/QOc3gBpnPgtwECVORwzO+w5rNY9BBLNRQE6AMtLtl foreman-proxy.pnq2.redhat.com"], "remote_execution_ssh_user"=>"rex", "remote_execution_effective_user_method"=>"sudo", "remote_execution_connect_by_ip"=>true}