Add GCP validation for project and shared_vpc_project_number

[okrause]

This PR contains three commits.

1. Fix typo (montly -> monthly) in variable name. Cosmetic

2. Add validation for "project" and "shared_vpc_project_number" attributes
   On GCP, users often confuse project_ID (mostly used within GCP) and project_number (which the CVS API uses). This led to multiple annoyances when users specified the alpha-numerical project_ID , instead of the numerical project_number. When doing so, the error message is less than helpful:

netapp-gcp_volume.gcp-volume: Creating...

Error: invalid character '<' looking for beginning of value

  on volume.tf line 3, in resource "netapp-gcp_volume" "gcp-volume":
   3: resource "netapp-gcp_volume" "gcp-volume" {

This commit adds a validation to "project" and "shared_vpc_project_number" to be numeric. Since project_id needs to start with a lowercase letter, this will prevent users from specifying a project_id and will return an helpful error message. The commit misses tests to test the validation. Manual tests did work.

3. Mark the password field of Active Directory username/password as sensitive. Terraform will stop printing its content, but will print "(sensitive value)" instead.

Additionally, my editor (VSCode) enforces some formatting rules automatically. Some lines got cleaned up.

[lonico]

an --> a

[lonico]

is there a convention here, I see that these lines keep moving around.

[okrause]

Likely, but not directly enforced by myself. I am using VScode and it does a lot of formatting and adding imports automatically. See here: https://github.com/golang/vscode-go/blob/master/docs/features.md#format-and-organize-imports
Looks like it is eventually calling "goimport", which manages the import lines and organises them in a defined GO way.