Add libssl-dev/openssl-devel to the list of dev packages dependencies (#446)
Add libssl-dev/openssl-devel to the list of dev packages dependencies (#446)

* added `libssl-dev` as a dependency to the `libthemis-dev` DEB packages
* added `openssl-devel` as a dependency to the `libthemis-devel` RPM package
* renamed `DEBIAN_*` -> `DEB_*` variables to be consonant to `RPM_*` variables and more accurately reflect the essence

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