(maint) refactor keytool provider specs

Removed instances of mocha `#with` blocks and replaced them with mocha
parameter matchers.

Add in additional method stubs for `command_keytool` and
`command_openssl` to stub out command paths.

Convert instance variable for fixture data into `let` block.
Merge pull request #22 from adrienthebo/maint-correct_rspec_stubbing

(maint) refactor keytool provider specs