Added support to create databricks_user on the account level

[nfx]

This PR fixes #818

provider "databricks" {
  host = "https://accounts.cloud.databricks.com/"
  account_id = "aaaaaa-bbbb-cccc-dddd"
}

resource "databricks_user" "me_admin" {
  user_name = "me@example.com"
}

[codecov]

Codecov Report

Merging #849 (7a6db83) into master (0649acf) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #849      +/-   ##
==========================================
- Coverage   83.05%   83.03%   -0.03%     
==========================================
  Files          92       92              
  Lines        8203     8204       +1     
==========================================
- Hits         6813     6812       -1     
- Misses        892      893       +1     
- Partials      498      499       +1     

Impacted Files	Coverage Δ
common/http.go	84.80% <0.00%> (-0.66%)	⬇️