fix(computes): editing a compute keeps its password unless a new one is given

The TCP and TLS compute forms never show the stored password, so the
field is empty on every edit. Saving the form without typing the
password again replaced it with nothing, and the next connection to the
host failed to authenticate.

An empty password field on an edit now keeps the stored password; a
new value replaces it.

computes/forms.py

@@ -7,7 +7,15 @@ from computes.models import Compute
 from .validators import validate_hostname
 
 
-class TcpComputeForm(forms.ModelForm):
+class KeepPasswordMixin:
+    """The form never shows the stored password: left empty on an edit, it
+    stays as it is."""
+
+    def clean_password(self):
+        return self.cleaned_data["password"] or self.instance.password
+
+
+class TcpComputeForm(KeepPasswordMixin, forms.ModelForm):
     hostname = forms.CharField(validators=[validate_hostname])
     type = forms.IntegerField(widget=forms.HiddenInput, initial=CONN_TCP)
 
@@ -26,7 +34,7 @@ class SshComputeForm(forms.ModelForm):
         exclude = ["password"]
 
 
-class TlsComputeForm(forms.ModelForm):
+class TlsComputeForm(KeepPasswordMixin, forms.ModelForm):
     hostname = forms.CharField(validators=[validate_hostname])
     type = forms.IntegerField(widget=forms.HiddenInput, initial=CONN_TLS)