ADE: "TypeError: cannot concatenate 'str' and 'int' objects" in check_util.py

Saw the following error in the log when trying to encrypt a RHEL 7.3 / *Standard DS1 v2* VM,
I have submitted a pull request to fix it: #644

**extension.log:**
```
2018/09/18 02:01:37 [Microsoft.Azure.Security.AzureDiskEncryptionForLinux-1.0]: 9933: [Info] PRECHECK: Exception thrown during precheck
2018/09/18 02:01:37 [Microsoft.Azure.Security.AzureDiskEncryptionForLinux-1.0]: 9933: [Info] Traceback (most recent call last):
2018/09/18 02:01:37 File "/var/lib/waagent/Microsoft.Azure.Security.AzureDiskEncryptionForLinux-0.1.0.999322/main/handle.py", line 482, in enable
2018/09/18 02:01:37 if cutil.is_precheck_failure():
2018/09/18 02:01:37 File "/var/lib/waagent/Microsoft.Azure.Security.AzureDiskEncryptionForLinux-0.1.0.999322/main/check_util.py", line 99, in is_precheck_failure
2018/09/18 02:01:37 if self.is_insufficient_memory():
2018/09/18 02:01:37 File "/var/lib/waagent/Microsoft.Azure.Security.AzureDiskEncryptionForLinux-0.1.0.999322/main/check_util.py", line 52, in is_insufficient_memory
2018/09/18 02:01:37 self.logger.log('WARNING: total memory [' + memtotal + 'kb] is less than 7GB')
2018/09/18 02:01:37 TypeError: cannot concatenate 'str' and 'int' objects
2018/09/18 02:01:37
```