Check that envvars are valid identifiers when using encrypted strings

[mikljohansson]

When Secretary decrypts something it outputs a scriptlet that is sourced into the env, to overwrite encrypted env vars with decrypted ones. Meaning that env vars with encrypted substrings like rabbitmq.password won't work since they contain dots. This change ensures the problem is caught with a user readable error

[codecov-io]

Current coverage is 55.36%

Merging #8 into master will increase coverage by +0.92% as of 57ead2f

@@            master      #8   diff @@
======================================
  Files            8       8       
  Stmts          461     466     +5
  Branches        70      70       
  Methods          0       0       
======================================
+ Hit            251     258     +7
+ Partial         31      30     -1
+ Missed         179     178     -1

Review entire Coverage Diff as of 57ead2f

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